Colour Perception

Colour exists in the brain

Colour is not a property of an object. It is a perception — the result of the processing of light by the visual system. Light falls on an object, is reflected, and reaches the retina. Three types of cones — sensitive to short (S), medium (M), and long (L) wavelengths respectively — convert that light into signals.

These signals are processed in two opponent colour channels: a red-green axis (L−M) and a blue-yellow axis (S−(L+M)). Only after this processing in the visual cortex does what we call colour emerge. This means that two people looking at the same object can have different colour experiences — not because one of them is mistaken, but because their visual system processes the same signal differently.

How sharp is human colour discrimination?

Colour discrimination is not a binary property but a sensitivity that varies by colour context, by observer, and by moment. Paramei & Oakley (2014) studied 291 observers aged 10 to 88 using the Cambridge Colour Test. They found a consistent pattern: sensitivity peaks around age 27–30 and then gradually declines.

The decline is not uniform. The blue-yellow channel (tritan axis) is the most vulnerable: the discrimination threshold in this channel increases early in adulthood, while the red-green channel remains stable longer. This has a practical consequence: people over forty have more difficulty distinguishing colours in the blue-purple-green region.

Yellowing lenses and individual variation

As people age, the eye lens yellows. This yellowing process dampens short wavelengths (blue light) and directly affects colour matching. Shi et al. (2023) studied this in 100 observers aged 8 to 80 and found that inter-observer variation increases strongly with age.

The CIE 2006 standard, based on averages, therefore underestimates the reality for individual colour assessors in industry. An observer aged 55 and one aged 25 can assess identical colours differently in good lighting — not through inattention, but due to physiological differences in the optical system.

Three layers of colour competency

Professional colour competency consists of three separate skills. Discrimination is perceiving that two stimuli differ from each other. Attribution is recognising which property changed — hue, lightness, or chroma. Communication is correctly naming that difference using professional terminology.

Existing clinical instruments — the FM100, the Cambridge Colour Test, the CAD test — measure discrimination only. The National Research Council (1981) explicitly stated that no single clinical instrument can predict the ability to recognise, name, and communicate colour in variable conditions. Attribution and communication are measured by none of these instruments.

Colour perception is trainable

The human visual system is adaptable. Targeted practice demonstrably improves the precision of colour discrimination, particularly in the dimensions being trained. Perge (2015) showed a significant effect (p < 0,001) of digital colour training on discrimination scores.

Training effectiveness depends on the quality of the stimuli. Training modules using HSB or sRGB produce stimuli with unequal difficulty levels across the colour spectrum. ColorAptitude uses OKLCH-based stimuli with perceptually uniform spacing (Ottosson, 2020), ensuring the difficulty level is consistent across all colour regions.

References

• Kinnear, P. R., & Sahraie, A. (2002). New FM100 norms. British Journal of Ophthalmology, 86(12), 1408–1411.
• Kotterink, M. (2026). A digital platform for professional colour competency. Stichting Xconea.
• National Research Council. (1981). Procedures for Testing Color Vision. National Academy Press.
• Paramei, G. V., & Oakley, B. (2014). Variation of color discrimination across the life span. JOSA A, 31(4), A375–A384.
• Perge, E. (2015). Digital colour training effectiveness. University of Debrecen.
• Shi, L., et al. (2023). Individual color matching functions. Journal of Vision.