Colour theory and colour management: mixing, spots, ICC profiles and densitometry
What the exam tests
Colour questions test additive versus subtractive mixing, why black exists in CMYK, when a spot colour is unavoidable, what an ICC profile does, and how to read a densitometer or a ΔE value.
Mixing models
- Additive (RGB): light sources. R + G = yellow; all three = white. Screens, cameras, scanners.
- Subtractive (CMY): pigments filter light. C + M = blue; C + M + Y ideally black but in practice a muddy brown, so K is added for density, text sharpness and lower ink cost.
- Gamut: RGB displays exceed CMYK print. Saturated orange, deep blue and metallics need spot inks or extended gamut (CMYK plus orange, green, violet).
ICC colour management
- A profile describes a device's colour behaviour against the Profile Connection Space (CIELAB/XYZ): source profile → PCS → destination profile.
- Rendering intents: perceptual (compresses the whole gamut, good for photographs), relative colorimetric (maps white to paper white, keeps in-gamut colours, usual for proofing), absolute colorimetric (simulates the target paper, for contract proofs), saturation (charts and graphics).
- Assign the profile, do not merely embed and ignore it. Soft-proof on a calibrated monitor at D50, around 120 cd/m².
Measurement
- Reflection density D = log10(1/R); the paper base is measured and subtracted.
- Dot area by Murray–Davies: a = (1 − 10^−Dt) ÷ (1 − 10^−Ds), with tint and solid densities corrected for paper.
- TVI (dot gain) is printed dot area minus nominal. ΔE (CIEDE2000) below about 2 is invisible to most observers; 2–3 is usually acceptable in production.
Worked example
A 70 % tint patch reads 0.65, the solid 1.30, paper 0.05.
- Corrected tint Dt = 0.65 − 0.05 = 0.60; solid Ds = 1.30 − 0.05 = 1.25.
- a = (1 − 10^−0.60) ÷ (1 − 10^−1.25) = (1 − 0.251) ÷ (1 − 0.056) = 0.749 ÷ 0.944 = 0.793.
- Printed dot area is 79 % against 70 % nominal, so dot gain is 9 %: normal for uncoated, high for coated sheetfed.
Traps that cost marks
- Treating K as just another colour instead of a density and economy decision.
- Calculating dot area without subtracting the paper reading.
- Using absolute colorimetric when the client wants a brand colour matched on white stock.
- Assuming a spot colour can be built in CMYK without checking the gamut.