How to make a paint test cardboard

The test cardboard is the ultimate arbiter that tells you whether the colour will be right before you start work on the actual panel.

A colour viewed on a screen, in a paper colour chart or seen in a tin tells you nothing about how it will look once applied and clearcoated on the car. That’s exactly what the test cardboard is for: to replicate the entire paint system in miniature, place it next to the part to be matched, and check the result in daylight. It takes just fifteen minutes and saves you from having to redo an entire side panel or bonnet. This tutorial explains, using the method followed at the Carross workshop, how to produce a reliable test panel, how to check the opacity and how to choose the right shade. To find the exact reference for the car, use the ‘Find car colour code’ tool.

Tech’Team tip

The game-changer: the 2-in-1 test coat.

Even with the same metallic or pearlescent formula, the finish varies depending on how the base coat is applied. This workshop tip allows you to test two finishes on a single test cardboard. First, apply your base coat until opaque across the entire cardboard, then immediately apply your topcoat ‘wet-on-wet’ over the whole surface. Leave to dry completely. Once the base coat is thoroughly dry, mask off half the panel widthways (never lengthways, to preserve the black-and-white opacity markings on each area), then apply a top coat ‘dry’ to the uncovered half. Remove the masking and apply a clearcoat: you can see on a single test panel which method best directs the particles and adheres perfectly to the car.

The essentials in a nutshell. Test cardboard is a neutral surface onto which the complete system — primer, base coat and clearcoat — is applied to validate a colour before painting the actual part. It is compared, once dry and clearcoated, in daylight, against the adjacent bodywork. If it’s a perfect match, we paint; otherwise, we try the next variant of the colour code — a step that we at Carross never skip before blending.

1. Diagnosis first and foremost

• Colour to be confirmed before a repair — a test panel applied with the full system (including the primer shade), compared dry and with a clearcoat applied against the adjacent part, which has been previously cleaned and polished
• If there is any uncertainty after reading the spectrophotometer or consulting the colour chart — produce a test panel only for the closest variants, in order to compare them side by side in daylight
• Special-effect colours (metallic, pearlescent, three-coat) — a test panel is mandatory, applied using the same nozzle, the same pressure and the same spray pattern as on the car
• Doubts regarding theoretical coverage or an old pot — produce a test panel with black and white opacity markers before applying the paint to the part

2. The equipment required

• Metal test cards with opacity markers
• Bodywork degreaser
• Lint-free wiping cloths
• Primer
• Car body colour paint
• Air-drying car body clearcoat
• High-gloss spray clearcoat
• Colour-matching lamp (daylight)

3. Step-by-step guide

1. Prepare the test cardboard and the reference part. Degrease a metal test cardboard and apply the recommended primer shade. Clean and polish the adjacent area on the car to allow comparison against a clean, non-oxidised clearcoat.
2. Apply the basecoat. Spray the colour-matched paint using the same spray gun, pressure, distance and number of coats as on the vehicle, until the opacity mark is covered. For a metallic or pearlescent colour, finish with a positioning coat.
3. Allow the solvent to evaporate. Wait until the matt basecoat has dried completely (even matt finish) before applying the clearcoat, otherwise the metallic or pearlescent pigments may be drowned out and the colour distorted.
4. Apply theclearcoat. Apply the bodywork clearcoat to give the colour its final depth and gloss.
5. Dry and label. Once the clearcoat is dry, note the colour code, the chosen variant, the base shade, the number of coats and the date on the back of the colour chip so that you can file the test in your workshop colour chart.
6. Compare in daylight. Place the test panel edge-to-edge against the neighbouring polished part, either outdoors or under a colour-matching lamp. Assess the colour from two or three angles.
7. Make a decision. If the sample blends in from all angles, approve the formula and proceed with painting. If a slight discrepancy remains, adjust the application technique (pressure, distance), test the next variant, or plan for a blended transition on the adjacent element.

4. Common mistakes to avoid

• Comparing an unvarnished base: the perceived colour and depth change radically once a clearcoat is applied; any assessment based on a matt basecoat is inaccurate
• Neglecting the base colour or opacity: ignoring the recommended primer shade or failing to cover the black-and-white test mark will distort the final result, particularly with low-coverage colours (white, reds, yellows, pearlescent shades)
• Judging under standard workshop lighting: standard fluorescent lights cause metamerism. Always compare under daylight or a colour-matching lamp
• Do not compare against a dirty or dull surface: holding your colour chart against an oxidised or micro-scratched clearcoat misleads the eye. The reference surface must be cleaned and polished
• Paint the colour swatch differently from the part: changing the pressure, distance or speed of application alters the orientation of the aluminium or pearlescent particles and distorts the effect of the colour
• Only looking at the test panel from the front: on a metallic or pearlescent finish, a colour difference often gives the painter away when viewed at an angle (the ‘flop’), even though it looked fine from the front
• Forgetting to label the test panel: failing to note the code, variant, pressure and number of coats on the back renders the panel unusable for your future projects

5. How to choose your products

Type of colour: opaque → one colour swatch is sufficient to check the colour and coverage; metallic, pearlescent or three-coat → a colour swatch is essential, to be assessed from the front and at a grazing angle (flop)

Number of variants: one colour chip per selected variant (or one suggested by the spectrophotometer), compared side by side on the adjacent glossy surface

Swatch substrate: metallic with black and white markings (chequered pattern or stripes) to check opacity, prepared with the same shade of primer as the component

Comparison lighting: always in natural daylight or under a colourimetric lamp, never under standard workshop lighting

Carross tip: never make a decision about a colour match without a colour chart, and file away each chart with notes on the back to create your own workshop colour chart and save time on future jobs

6. FAQ

Why should the colour swatch be clearcoated before comparison?

Because the clearcoat alters the refraction of light and therefore the perceived colour. When the system specifies a clearcoated base, the comparison must be made against the finished appearance: the base and the clearcoat do not have the same surface finish or the same gloss level. A water-based or solvent basecoat, once dry, is matt, dull and appears lighter. The clearcoat ‘wets’ the pigments, darkens the colour and brings out the lustre of metallic or pearlescent particles. Comparing an unvarnished base coat to a varnished bodywork is a beginner’s mistake that is guaranteed to result in a colour mismatch. Once the base coat has been fully matted, you should therefore always apply the clearcoat (using a spray gun or a gloss clearcoat spray to save time) in accordance with the manufacturer’s instructions and allow it to dry for the required time before assessing the result.

Under what lighting should a test cardboard be compared?

In natural daylight or using a colour-matching lamp (artificial daylight), on a clean, polished surface. Conventional workshop lighting (standard neon lights) causes metamerism: a colour may appear identical indoors but look completely different outdoors. Always assess the colour chart from several angles, as a colour with a special effect changes appearance when viewed head-on versus at a low angle (the ‘flop’).

Is a sample required for every variant of a colour code?

Only for those that remain in the running after an initial selection. Some colour codes have up to ten variants: it is unthinkable to paint them all. We first narrow down the choice directly on the vehicle using the paper colour chart (colour swatches) or a spectrophotometer to select the one that comes closest. If there is still some uncertainty between two or three very similar shades, we then prepare a sample panel for each shortlisted variant, applied in the same way, and line them up side by side against the part to be matched. The one that blends in best both from the front and at an angle is the right one. By noting the reference numbers on the back and keeping these test panels in your workshop colour chart, you won’t need to make them again the next time this colour code comes up.

Is a test cardboard useful for a simple opaque colour?

Yes, for three reasons: to check the exact shade (an ice white may have a yellow, blue or grey depending on the variant), to avoid metamerism (common with opaque colours, which may look perfect under the workshop’s neon lights but change completely once in daylight), and, above all, to check the theoretical coverage. Opaque pigments such as yellow, orange or red often provide very poor coverage: the test panel, with its black and white reference marks, allows you to determine in advance whether you’ll need 2, 3 or 4 coats to ensure the primer doesn’t show through.

Updated: 29/10/2026

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