What Does Colour Blindness Actually Look Like? Not Black and White

Normal human colour vision depends on three types of cone cell in the retina, each containing a pigment most sensitive to a different range of wavelengths, conventionally described as long, medium and short. No cone identifies a colour by itself, since each responds across a broad range, and colour is perceived from the ratio of responses between the three types, which the visual system compares. Removing or altering one type removes one dimension of that comparison, so certain colours that previously produced distinct ratios now produce the same one and become indistinguishable. Which colours those are depends on which cone type is affected, which is why the different forms of the condition have specific and predictable confusions rather than a general loss.

The types differ in which cone is affected and in how common they are:

• •Anomalies of the long or medium cones, which shift sensitivity rather than removing it, and are the commonest forms
• •Absence of one of those cones entirely, which produces stronger confusion between reds and greens
• •Blue and yellow forms affecting the short cone, which are far rarer and affect both sexes equally
• •Complete absence of colour vision, which is very rare and comes with poor acuity and light sensitivity
• •The common forms affect roughly one man in twelve and one woman in two hundred
• •The genes for the long and medium pigments sit on the X chromosome, which explains the sex difference

Descriptions are difficult because a person with the condition has no experience of the alternative, and simulations shown to people with normal vision are approximations of an experience nobody can share directly. What can be stated is which discriminations fail. Red and green of similar brightness become difficult, as do brown and green, orange and red, and blue and purple. Colours are not absent, since two of the three dimensions remain and the world looks fully coloured. Brightness differences remain intact and are used heavily as a substitute cue. Most people with the common forms manage without difficulty in daily life and learn the colours of familiar objects, which is why many are not diagnosed until tested and why some discover it in adulthood.

Several tests exist and they answer different questions. Plates with coloured dots forming a figure visible to some observers and not others are the familiar screening method, quick and effective for detecting the common forms and poor at grading severity. Arrangement tests, where a person orders a set of coloured caps into a sequence, measure how severe the confusion is and which axis it lies on, and are used where a precise result matters. An instrument that asks the observer to match a mixture of red and green light to a yellow reference is the reference standard and distinguishes the forms precisely. Testing children matters more than it is done, since a child who cannot distinguish materials used in teaching may be marked down for something nobody has identified.

The practical consequences fall on whoever designs anything that carries information in colour. Using colour as the only means of distinguishing categories fails for a substantial part of the audience, which is why accessibility guidance requires a second cue such as shape, position, label or pattern. Red against green is the worst combination and appears constantly in status indicators, maps and charts. Colour scales running from red to green for data are widely used and widely criticised, with alternatives designed to be distinguishable by everybody now available and increasingly standard in scientific publishing. Certain occupations impose colour vision requirements including some aviation, maritime and rail roles, where signal identification matters and where the requirement is contested in some cases.

Colour is perceived from the ratio of responses between three cone types, so altering one removes a dimension of comparison and makes specific pairs indistinguishable rather than removing colour. Red against green of similar brightness is the classic difficulty. The genes sit on the X chromosome, which is why the common forms affect roughly one man in twelve and far fewer women.