Why Does the Same RAL Colour Look Different on Two Batches of Prepainted Aluminium Coils?

Why Does the Same RAL Colour Look Different on Two Batches of Prepainted Aluminium Coils?

A customer orders two batches of colour-coated aluminium coil several months apart. Both orders specify the same alloy, thickness, coating system and RAL colour number. The first batch is used for a building facade, while the second batch arrives later for an extension project.

During installation, the customer notices something unexpected: the two batches do not look exactly the same.

One appears slightly darker, another looks more yellow, or the surface seems to have a different level of brightness. The difference may be difficult to notice when the coils are inspected separately, but it becomes obvious when panels from the two batches are installed next to each other.

This is a common challenge in the prepainted aluminium industry.

A RAL colour number identifies a standard colour reference, but it does not automatically guarantee that every coating batch will have identical visual characteristics. The final appearance is influenced by the paint formulation, pigment dispersion, coating thickness, gloss, substrate, curing conditions and lighting environment.

For manufacturers, maintaining consistent colour requires control over the entire coil coating process. For buyers, understanding these variables is important when ordering multiple batches for the same project.

1. What Does a RAL Colour Number Actually Guarantee?

1.1 Understanding the RAL Colour System

RAL is a widely used colour reference system in architectural, industrial and decorative applications.

When a customer requests RAL 9010, RAL 9005 or RAL 7016, the colour code provides a recognised reference for the intended colour appearance.

However, a RAL code is not a complete manufacturing specification for prepainted aluminium.

It does not independently define:

• The exact paint formulation used by a coating manufacturer.

• The pigment combination and dispersion quality.

• The coating thickness.

• The gloss level.

• The surface texture.

• The curing conditions.

• The acceptable batch-to-batch colour tolerance.

Two paint manufacturers may develop different formulations to achieve a similar RAL colour. Even when both products carry the same colour reference, their final appearance may not be identical.

1.2 Why a RAL Colour Card Is Not Always Enough

A printed colour card is useful for initial colour selection, but it should not be treated as the only acceptance standard for a large aluminium coil order.

There are several reasons.

First, the colour card and the finished aluminium product may use different materials and surface finishes. A printed paper reference does not reflect light in exactly the same way as a coated metallic surface.

Second, colour cards may have different gloss levels. A matt version of a colour can appear different from a glossy version, even when both carry the same colour designation.

Third, physical colour cards can change over time due to storage conditions, light exposure and ageing.

For important projects, a physical coated aluminium sample approved by both the buyer and supplier is a more useful reference than a RAL number alone.

1.3 Same Colour Code Does Not Always Mean Same Visual Appearance

Consider a project that requires RAL 9006 metallic silver.

Two manufacturers may both claim to supply RAL 9006, but the final appearance can vary because of differences in metallic pigments, particle orientation, gloss and coating formulation.

The same issue can occur with solid colours, although metallic and pearlescent finishes are generally more sensitive to viewing angle and surface orientation.

For this reason, colour selection should be followed by sample approval and clearly defined acceptance criteria.

2. The Main Reasons Why Two Batches of Prepainted Aluminium Look Different

2.1 Different Paint Batches and Pigment Formulations

One of the most direct causes of colour variation is a difference in the paint itself.

Coil coating paint contains several components, including resins, pigments, solvents and additives. Pigments provide the colour, while the resin system forms the coating film.

If the pigment concentration, pigment combination or dispersion changes between paint batches, the final colour may also change.

For example, a small variation in pigment balance can make a grey coating appear slightly warmer or cooler.

Why Pigment Dispersion Matters

Pigments must be distributed evenly throughout the paint.

If dispersion is inconsistent, some areas may reflect or absorb light differently from others. This can result in uneven colour or differences between production batches.

The problem is particularly noticeable in colours that are sensitive to small formulation changes, such as:

• Light grey.

• Cream and ivory.

• Beige.

• Pale blue.

• Pastel colours.

• Certain deep reds and greens.

A reliable paint supply and controlled formulation are therefore important for maintaining colour consistency.

2.2 Different Coating Thicknesses

Coating thickness affects more than just the protective performance of prepainted aluminium. It can also influence the perceived colour and gloss.

When the coating thickness changes, the way light interacts with the coating surface may change as well.

A thicker or thinner coating can produce a slightly different visual appearance, especially when the coating formulation contains transparent pigments, special-effect pigments or multiple layers.

For example, two batches may use the same paint formulation but have different dry film thicknesses due to variations in application conditions.

The resulting panels may not look identical even if the RAL reference is unchanged.

2.3 Differences in Gloss Level

Gloss is one of the most underestimated factors in colour matching.

Two samples may have very similar measured colour coordinates but still look different because one surface is more reflective than the other.

A high-gloss coating reflects more light in a concentrated direction, while a matt coating scatters light more diffusely.

This can change how people perceive the same colour.

How Gloss Changes Colour Appearance

For this reason, colour and gloss should be specified together.

A customer who approves a matt RAL sample should not assume that a high-gloss version of the same colour will look identical.

2.4 Different Curing Conditions During Production

Coil coating is a continuous industrial process. After paint application, the aluminium strip passes through an oven where the coating is cured.

The final appearance can be influenced by the curing conditions.

Important variables include:

• Actual metal temperature.

• Oven temperature profile.

• Line speed.

• Time spent in the curing zone.

• Coating formulation.

• Cooling conditions after curing.

If these conditions change between production runs, the coating may develop slightly different colour or gloss characteristics.

The actual metal temperature is particularly important. The oven setting alone does not fully describe the thermal conditions experienced by the aluminium strip.

Why the Same Paint Can Produce Different Results

Even when the same paint batch is used, changes in production conditions can affect the final coating film.

A difference in line speed may change the time the strip spends in the oven. A change in metal temperature may influence the curing reaction. Both can affect the final appearance and coating properties.

This is one reason why maintaining stable production parameters is essential when manufacturing repeat orders.

2.5 Different Aluminium Substrates

The aluminium substrate can also influence the appearance of the finished coating.

Different alloys, surface conditions and pretreatment characteristics may affect how the coating forms and how light is reflected from the finished surface.

For example, a customer may order one batch using 3003 aluminium and a later batch using 3105 aluminium, assuming the same RAL colour will produce exactly the same appearance.

Although both alloys can be used for colour-coated products, differences in substrate surface characteristics and production conditions should be considered.

The same applies to different thicknesses, surface roughness and pretreatment systems.

For large projects, keeping the substrate specification consistent is an important part of colour control.

2.6 Different Surface Textures and Finishes

A smooth surface and a textured surface can produce different visual impressions even when their colour formulation is similar.

Embossed, brushed, sand-textured and other decorative finishes change the way light interacts with the surface.

This is particularly important for:

• Wood-grain aluminium.

• Brushed aluminium.

• Embossed aluminium sheets.

• Stone-pattern decorative panels.

• Metallic and pearlescent finishes.

A colour measurement method designed for smooth, solid-colour surfaces may not fully describe the appearance of these special finishes.

2.7 Metamerism: Why the Same Two Samples Look Different Under Different Lights

Metamerism is an important reason why two colour samples may appear to match under one lighting condition but look different under another.

This happens when two coatings have different spectral reflection characteristics. They may look similar under daylight but become visibly different under artificial lighting.

For example:

• Under natural daylight, two grey panels may appear almost identical.

• Under warm indoor lighting, one may look slightly yellow.

• Under cool fluorescent lighting, the difference may become more noticeable.

This phenomenon can create confusion when customers inspect samples in an office and later compare the finished aluminium panels outdoors.

How to Reduce Metamerism Problems

For projects where colour matching is important, the buyer and supplier should agree on the reference lighting conditions.

Common assessment conditions include:

• D65: Standard daylight simulation.

• Illuminant A: Represents warm incandescent lighting.

• TL84: Commonly used for certain commercial fluorescent lighting comparisons.

For exterior architectural applications, daylight assessment is often particularly relevant. However, if the finished panels will be viewed under specific artificial lighting, that condition should also be considered.

3. How Can Manufacturers Control Colour Consistency Between Different Batches?

Colour consistency cannot be achieved simply by selecting the correct RAL number. It requires a repeatable production and inspection process.

3.1 Keep a Physical Master Sample

A physical master sample should be approved before mass production.

It should represent the actual coated aluminium product rather than relying only on a printed colour card.

The reference sample should identify:

• RAL or custom colour reference.

• Aluminium alloy and temper.

• Coating system.

• Coating thickness.

• Gloss level.

• Surface finish.

• Production direction, where relevant.

The sample should be properly labelled and stored to reduce the risk of accidental damage, contamination or ageing.

For repeat orders, the supplier should compare the new production sample against the approved master.

3.2 Use a Spectrophotometer to Measure Colour Difference

A spectrophotometer measures how a surface reflects light at different wavelengths and converts the result into colour coordinates.

For many solid-colour coatings, measurements are expressed using the CIELAB colour space:

• L*: Lightness, from darker to lighter.

• a*: Position between green and red.

• b*: Position between blue and yellow.

The difference between two sets of colour coordinates can be expressed as ΔE.

A smaller ΔE generally indicates a closer colour match under the specified measurement conditions.

However, there is no single universal ΔE acceptance limit suitable for every colour, finish and application. The acceptable tolerance should be agreed between the buyer and supplier before production.

Why Measurement Conditions Must Be Consistent

Two ΔE readings cannot be compared reliably if the measurement conditions are different.

The parties should agree on:

• The colour difference formula, such as CIE 1976 ΔE*ab.

• Illuminant.

• Standard observer angle.

• Instrument geometry.

• Measurement aperture.

• Sample preparation and surface direction.

• Number and location of measurement points.

For textured, metallic and pearlescent coatings, visual assessment and specialised measurement methods may also be required.

3.3 Control Paint Formulation and Raw Material Batches

Paint formulation should be controlled as carefully as the coating process itself.

For repeat orders, manufacturers should maintain traceability of the paint batch and, where practical, use the same approved formulation and pigment combination.

This reduces unnecessary changes between production runs.

If a paint formulation must be changed, the new material should be evaluated against the approved reference before it is released for production.

3.4 Maintain Stable Coating Thickness and Curing Conditions

A consistent production process helps reduce differences in colour and gloss.

Manufacturers should monitor:

• Coating application settings.

• Wet and dry film thickness.

• Line speed.

• Actual metal temperature.

• Oven conditions.

• Cooling conditions.

• Surface appearance after curing.

These parameters should be recorded so that any variation can be investigated later.

3.5 Compare Samples From Different Batches Before Shipment

For large orders delivered in multiple batches, it is better to compare the batches before they are shipped rather than waiting until the material reaches the installation site.

A practical process includes:

1. Take representative samples from each production batch.

2. Compare the samples with the approved master.

3. Measure colour difference under the agreed conditions.

4. Check gloss and surface finish.

5. Conduct visual comparison under standardised lighting.

6. Record the results and confirm whether the batches can be used together.

This process is particularly useful for large facades, roofing systems and other projects where panels from different production dates will be installed next to each other.

4. How Should Buyers Specify Colour Requirements When Ordering Prepainted Aluminium?

Many colour complaints start before production because the purchase order contains only a RAL number.

For a standard stock product, this may be sufficient for an initial quotation. For a project involving multiple batches, however, a more detailed specification is necessary.

4.1 Do Not Specify Only the RAL Number

Instead of writing only:

"Colour: RAL 7016"

A more complete specification should include:

• RAL colour number and reference edition, where relevant.

• Approved physical coated aluminium sample.

• Coating system, such as PE or PVDF.

• Gloss level.

• Surface finish.

• Colour tolerance.

• Measurement method.

• Inspection lighting conditions.

This gives the supplier a much clearer target.

4.2 Agree on the Acceptable ΔE Tolerance

The buyer and supplier should agree on a colour tolerance before mass production.

For example, a project may specify a maximum ΔE value against an approved master sample, measured under agreed conditions.

However, a value such as ΔE ≤ 1.0 should not be treated as a universal requirement. The appropriate tolerance depends on the colour, coating, measurement method and visual expectations of the project.

Light colours, dark colours and special-effect finishes may require different assessment approaches.

4.3 Keep One Production Batch for One Project Where Possible

If a project requires several coils of the same colour, producing them together can reduce the risk of batch-to-batch variation.

This is especially useful for:

• Large building facades.

• Roofing projects.

• Aluminium ceiling systems.

• Long decorative wall panels.

• Large appliance production runs.

If one production batch cannot cover the entire order, the buyer should ask the supplier to arrange the production sequence and compare the batches before shipment.

4.4 Plan for Future Reorders

A project may require additional aluminium months or years after the original order.

At that point, matching the original appearance becomes more difficult because the original material may have weathered, faded or changed in gloss.

Buyers should therefore keep:

• The original purchase order.

• The approved master sample.

• The original colour measurement report.

• The coating specification.

• The production batch information.

• Any available retained reference material.

These records help the supplier understand the original requirement and provide a closer match for future orders.

However, an exact visual match may not always be possible, particularly when the original panels have already been exposed to sunlight and weather.

5. What Happens When Colour Differences Are Not Controlled?

A small difference in colour may seem unimportant when inspecting a single aluminium coil. However, it can become a significant problem after fabrication and installation.

5.1 Uneven Colour Across Building Facades

On a large facade, panels from different production batches may be installed next to each other.

If the colour difference is noticeable, the building surface may appear patchy or inconsistent.

The problem is particularly visible when sunlight falls across panels at different angles.

5.2 Customer Complaints and Replacement Costs

When a colour mismatch is discovered after installation, the cost is not limited to the replacement aluminium.

Additional expenses may include:

• Removing installed panels.

• Reordering replacement material.

• Labour and transportation.

• Project delays.

• Additional inspection.

• Customer disputes.

This is why colour approval should be completed before production rather than after the material has been installed.

5.3 Problems During Large-Scale Manufacturing

Appliance manufacturers and industrial processors may purchase the same colour-coated aluminium repeatedly.

If the colour changes between deliveries, the difference may become visible between production batches of finished products.

For companies producing refrigerators, air-conditioning equipment, lighting housings or decorative panels, colour consistency is an important part of maintaining a uniform product appearance.

6. Practical Checklist Before Ordering Multiple Batches of Colour-Coated Aluminium

The following checklist can help buyers reduce the risk of colour differences.

A clear specification is not just a quality control document. It is also a way to prevent misunderstandings between the aluminium supplier and the customer.

7. Where Can Buyers Find Reliable Prepainted Aluminium Coil Suppliers?

When selecting a colour-coated aluminium supplier, buyers should consider more than the available RAL colour chart.

A reliable supplier should understand the relationship between paint formulation, substrate, coating application, curing conditions and final colour appearance.

The following companies are useful starting points for researching established prepainted aluminium and coil coating suppliers. This is a procurement reference list, not an independent ranking or a guarantee of identical colour performance across their product ranges.

7.1 Dingang – China

Dingang supplies colour-coated aluminium coil, sheet and related aluminium products for building materials, architectural decoration, household appliances and industrial applications.

Its product range includes different aluminium alloys, coating systems, colours and surface finishes.

For customers who need the same RAL colour across multiple orders, it is important to confirm the physical master sample, gloss, coating specification and acceptable colour tolerance before production.

Dingang can discuss customised colour-coated aluminium requirements according to the customer's dimensions, alloy, temper, application and surface appearance expectations.

Website: https://www.cnchangsong.com/

7.2 Arconic Architectural Products – France

Arconic Architectural Products, based in Merxheim, France, manufactures Reynolux® prepainted aluminium coils and sheets.

Its products are used in applications including facades, roofing, signage, interiors, transportation and industry.

For buyers comparing established European coil-coated aluminium suppliers, Arconic is a relevant company to consider. Its colour collections and coating options provide a reference for architectural aluminium projects.

Website: https://www.arconic.com/

7.3 Metalcolor – Switzerland

Metalcolor is a specialist aluminium coil coating company established in Switzerland in 1981.

The company offers a range of coil coating products, including custom colours, different gloss levels and decorative finishes.

Its production and quality control information also highlights the use of colour measurement and gloss inspection equipment.

Metalcolor is a useful reference for buyers looking for specialised aluminium coil coating and customised surface appearance solutions.

Website: https://metalcolor.ch/

7.4 What Should Buyers Compare Between Suppliers?

Before placing an order, buyers should ask suppliers the following questions:

1. Can you provide a physical coated aluminium sample for approval?

2. Can you retain the colour formulation and production records for repeat orders?

3. Do you measure colour using a spectrophotometer?

4. Can you provide the colour difference measurement conditions and results?

5. Can you control gloss and coating thickness between batches?

6. Can multiple coils for the same project be produced together?

7. How do you inspect colour consistency before shipment?

A supplier's ability to explain these details is a useful indication of how seriously it approaches colour quality control.

8. Frequently Asked Questions

8.1 Does the Same RAL Number Guarantee Exactly the Same Colour?

No. A RAL number identifies a standard colour reference, but different paint formulations, coating conditions, gloss levels and substrates can produce different visual results.

For important projects, approve a physical coated aluminium sample and define a colour tolerance.

8.2 What Is ΔE in Colour Measurement?

ΔE describes the numerical difference between two colours in a defined colour measurement system.

A lower value generally indicates a closer match. However, the result depends on the selected formula, illuminant, observer and measurement geometry.

The acceptable ΔE should be agreed between the buyer and supplier rather than assumed to be the same for every application.

8.3 Why Does the Same Colour Look Different Under Sunlight and Indoor Lighting?

This may be caused by metamerism.

Two coatings can look similar under one light source but different under another because their spectral reflection characteristics are not identical.

For important projects, visual comparison should be carried out under agreed lighting conditions.

8.4 Can Two Different Aluminium Suppliers Produce the Same RAL Colour?

Yes, but the final appearance may not be identical.

Different suppliers may use different paint manufacturers, pigment formulations, coating thicknesses and curing processes.

If panels from different suppliers will be installed together, physical sample approval and side-by-side comparison are strongly recommended.

8.5 How Can I Make Sure My Next Order Matches the Previous Batch?

Keep the original approved sample, colour measurement report and production specification.

When placing a repeat order, provide these references to the supplier and request a new sample for comparison before mass production.

8.6 Should I Order All Aluminium Coils for One Project at the Same Time?

If the project has strict colour consistency requirements, ordering the full quantity together is often the safer approach.

If separate deliveries are unavoidable, discuss batch planning, sample comparison and colour tolerance with the supplier before production begins.

Conclusion: A RAL Number Is the Starting Point, Not the Complete Colour Specification

The same RAL colour can look different between two batches of prepainted aluminium because the final appearance depends on much more than the colour code.

Paint formulation, pigment dispersion, coating thickness, gloss, curing conditions, aluminium substrate, surface texture and lighting can all influence the result.

For manufacturers, consistent colour requires stable production conditions and reliable inspection. For buyers, the most effective approach is to approve a physical master sample, define the colour tolerance and measurement conditions, and coordinate production batches before placing the order.

For large architectural and industrial projects, colour consistency should be discussed at the quotation stage, not after installation.

Dingang provides colour-coated aluminium coil and sheet solutions for different building, decorative and industrial applications. If your project requires a specific RAL colour, multiple production batches or long-term colour matching, share your aluminium specifications and approved colour reference with the team before production.

Visit Dingang – www.cnchangsong.com to explore colour-coated aluminium products and discuss your next project.