Oct 05, 2026 · 11 min read

PPGI Color Difference Causes: Batch to Batch Control

If your second PPGI batch does not match the first, the cause is almost always a change in one of five variables: pigment or tinting paste lot, paint line speed and oven temperature, primer shade, substrate colour, or the measurement method used to judge the difference. Colour is not a single property of a coil — it is the combined result of a paint film, a cured film thickness, a metallic-coated substrate, and a viewing condition. Change any one of them and the reflected light changes. The practical answer is to stop treating colour as a “paint code” and start treating it as a controlled specification: a reference standard, a measured tolerance, a defined measurement geometry, and a documented approval process. This article explains each drift mechanism and shows how to write a colour control clause that a supplier can actually be held to.

PPGI color difference causes batch to batch

Why PPGI colour drifts between batches

Pre-painted galvanised or galvalume coil (PPGI/PPGL) is a layered optical system. Light passes through the clear topcoat, reflects off pigment particles in the colour coat, and also reflects off the primer and the metallic coating beneath. Anything that alters the topcoat chemistry, the colour coat pigment, the primer opacity, or the substrate reflectance will shift the final colour reading, even if the paint supplier used the same nominal formula.

This is why two coils can carry the same RAL number and still look different side by side. RAL, Pantone and similar systems are colour designations, not manufacturing tolerances. They define an aim point, not an acceptable range. A supplier can be “within RAL” and still be visibly different from your retained sample.

The five main causes of batch-to-batch colour difference

1. Pigment and tinting paste lot variation

Inorganic and organic pigments are produced in lots. Even within a single pigment grade, tinting strength, particle size distribution and undertone can shift slightly from lot to lot. When a paint maker blends a new pigment lot into a colour paste, the mixed shade can move a small but visible amount. This is most noticeable in:

Whites and off-whites, where small yellowing or blueing shifts are obvious.
Light greys, beiges and pastels, where the human eye is very sensitive to hue drift.
Metallic and mica colours, where flake orientation and particle size dominate appearance.
Deep reds, oranges and some blues, where pigment chemistry is more complex.

Paint suppliers normally issue a certificate of analysis per batch. If your contract does not require batch traceability of the colour coat and a retained reference panel, you have no way to prove where a drift originated.

2. Line speed and oven temperature (cure window)

The coil coating line cures paint in a short, high-temperature cycle. Peak metal temperature (PMT) and dwell time control how completely the resin cross-links. Under-cure leaves a softer, sometimes slightly different-hued film; over-cure can yellow the topcoat, particularly with certain polyesters and polyurethanes. Line speed directly changes dwell time, so a speed change made for output reasons can change colour even with identical paint.

Practical consequences:

Coils run at the start or end of a campaign may sit at the edge of the cure window.
Different line speeds between two production runs can produce a measurable shift.
Oven temperature control drift, burner imbalance or a changed line load can alter PMT.

Buyers rarely specify cure conditions directly, because they are process parameters. Instead, specify the outcome: adhesion, solvent resistance (MEK double rubs), T-bend flexibility and colour, measured on the finished coil. That converts a process variable into a verifiable deliverable.

3. Primer shade and primer film thickness

The primer is not invisible. It contributes opacity, and in some systems it contributes colour. A primer that is slightly more yellow, or applied at a different dry film thickness, changes how much light reaches the metallic coating and how much is scattered back. Thinner primer over a shiny zinc surface can make the topcoat appear darker or greyer; thicker primer can lighten it.

If your supplier changes primer type or primer supplier between batches — often for cost or availability reasons — colour can move without any change to the topcoat.

4. Substrate colour and surface condition

The base metal matters. Hot-dip galvanised (GI coil) and zinc-aluminium coated substrates differ in brightness and spangle. Zinc spangle, skin-pass roughness, oil residue and oxidation all affect the diffuse reflection under a semi-transparent or thin coating system. A change in zinc coating weight, coating type, or the source of the base coil can therefore produce a colour difference that has nothing to do with the paint.

This is one reason colour consistency is easier to hold when the substrate specification is frozen: same coating type, same coating weight range, same surface treatment, same supplier route.

5. Measurement method and viewing conditions

A large share of “colour complaints” are actually measurement disagreements. Colour looks different depending on:

Illuminant (daylight, D65, cool white fluorescent, LED).
Observer angle and whether the sample is viewed at 45°/0° or with an integrating sphere.
Geometry: 45/0, d/8, or multi-angle for metallic colours.
Whether the coating is glossy or matte, and whether gloss itself has changed.
Instrument type, aperture size, calibration standard and operator technique.

Two parties can measure the same panel and report different ΔE values simply because they used different instruments or illuminants. Always fix the measurement geometry and illuminant in the contract.

Decision table: what to specify and why

Variable	What to specify	Why it matters	Verification method
Colour aim	Retained approved sample plus a named colour reference	Gives both parties one physical target	Signed and dated sample, stored indoors
Colour tolerance	ΔE limit and measurement geometry, e.g. 45/0, D65, 10°	Converts “looks different” into a pass/fail number	Spectrophotometer reading, recorded per coil
Gloss	Gloss level and tolerance at a stated angle, e.g. 60°	Gloss change alone can mimic a colour change	Gloss meter, same panel as colour
Paint system	Resin type, topcoat and primer, nominal DFT range	Controls cure behaviour and appearance	Coating thickness gauge, cross-section
Substrate	Coating type, coating weight range, surface treatment	Substrate reflectance affects final shade	Coating mass test, mill certificate
Cure / performance	Adhesion, T-bend, MEK resistance, pencil hardness	Indirectly guards the cure window	Standardised tests on finished coil
Traceability	Paint batch, coil ID, production date, line record	Lets you trace a drift to its source	Batch records supplied with coil

Note that specific numeric limits — ΔE, gloss units, DFT in microns, MEK rub counts — depend on your end use, your customer’s requirements and the paint system selected. Confirm the required values with your supplier and, where a standard is referenced, confirm the required edition and the exact contract values rather than relying on a general default.

How to write a colour control clause that actually holds

A colour clause fails when it says “colour must match the approved sample.” That is an aim, not a test. A clause that holds has five parts.

Part 1 — Define the reference

Name a physical approved sample, its date, who approved it, and how it is stored. State that the sample is the reference for both colour and gloss. If a named colour system such as RAL is used, state it as a designation in addition to the sample, not instead of it.

Part 2 — Define the measurement

Specify instrument type or geometry, illuminant, observer angle, aperture, and whether the reading is taken on the coated surface as-produced or after a defined cleaning. For metallic and pearlescent colours, specify multi-angle measurement, because single-angle readings do not capture flop.

Part 3 — Define the tolerance and the sampling plan

State the ΔE limit, the gloss tolerance, and how many readings per coil and per batch. State whether the tolerance applies to the batch mean, to individual readings, or both. State what happens to a coil that fails — rework, rejection, or concession with written agreement.

Part 4 — Freeze the inputs

List the inputs that may not be changed without written notice: paint supplier and product code, primer type, substrate coating type and coating weight range, base metal source route, and line. This is the clause that prevents silent substitution.

Part 5 — Define the evidence

Require that each shipment includes coil IDs, paint batch references, production date, and colour and gloss readings. Without records, a dispute becomes one person’s eye against another’s.

RFQ checklist for colour-critical PPGI orders

State the application and exposure: roofing, wall cladding, appliance, ceiling, or forming-intensive use.
Provide the colour reference: RAL or similar designation plus a physical approved sample.
State the required ΔE and gloss tolerance, or ask the supplier to propose values for your approval.
State the measurement geometry, illuminant and observer angle you will use for incoming inspection.
Specify the paint system: resin family, primer, nominal topcoat DFT range, and whether back coating is required.
Specify the substrate: GI or PPGL base, coating type, coating weight range, surface treatment.
State required mechanical and chemical performance: T-bend, adhesion, impact, MEK resistance, hardness.
State coil dimensions, ID, weight range, and packaging.
Ask whether the order will be produced in one campaign or split across campaigns.
Ask for paint batch traceability and colour measurement records with the shipping documents.
Ask how the supplier handles a colour dispute: retained samples, third-party testing, or joint re-measurement.
Request a dated written quotation that lists the specification, tolerances and validity period.

For reference on the base material side, see the galvanised steel coil page, and for the coated product range see PPGI coil.

Common mistakes buyers make

Treating RAL as a tolerance. A colour code defines an aim, not an acceptable range.
Approving a sample without recording it. An unsigned, undated or faded panel is not a reference.
Comparing coils under different light. Warehouse lighting and daylight give different readings.
Ignoring gloss. A gloss shift can read as a colour shift, especially on dark and saturated colours.
Changing substrate source to save cost. A different zinc coating or spangle can move the apparent shade.
Accepting a verbal “same as last time.” Without a frozen input list, the next batch can legally differ.
Measuring only at the coil end. Colour can vary across width and along length; sampling matters.
Leaving dispute resolution undefined. No retained sample and no records means no resolution path.

FAQs

Is a small colour difference between PPGI batches normal?

Minor variation is inherent to coil coating because pigments, cure conditions and substrates vary. Whether it is acceptable depends entirely on the tolerance you agreed. Without a stated ΔE and gloss limit, “normal” has no definition and disputes cannot be settled objectively.

Can I get an exact match to my previous batch?

An exact match is not a realistic contractual target. A close match within a defined tolerance is realistic, provided the paint system, primer, substrate and line conditions are held stable and the reference sample is retained. If you need visual consistency across a large project, order the full quantity in one campaign where possible.

Does changing from GI to PPGL base change the colour?

It can. Zinc-aluminium coatings differ in brightness and surface appearance from galvanised coatings, so the same paint system may read differently. If colour continuity matters, freeze the substrate specification as well as the paint specification.

How do I handle a colour dispute with a supplier?

Compare measurements using the geometry and illuminant named in the contract, on panels from both batches, under identical conditions. Check the retained approved sample. Review paint batch records and production records. If the parties still disagree, agree in advance on a third-party laboratory and accept its method as binding.

Does colour difference affect performance?

Not directly. Colour is an appearance property. However, a colour drift caused by under-cure or over-cure may coincide with changes in adhesion, flexibility or chemical resistance, so it is worth checking performance tests whenever colour moves. Actual service performance depends on substrate, metallic coating, paint system, environment, forming and maintenance.

How does colour specification affect price?

Tighter tolerances, special colours, small batch quantities, multi-campaign production and additional testing all add cost, because they reduce line efficiency and increase quality-control effort. Paint system choice, substrate grade and coating weight are usually larger cost drivers than colour itself. Because raw material and paint costs move, ask for a dated written quotation with a stated validity period rather than relying on an earlier figure.

If you are preparing a colour-critical PPGI or GI order and want the specification, tolerance and traceability points reviewed before you issue the RFQ, send your requirements, colour reference and application details through the ChinaCNU Steel contact page. Include your target ΔE and gloss tolerance if you have them, or ask for a proposed tolerance for your approval, and request a dated quotation covering the full specification.