Colour vision deficiency (colour blindness)

Say "colour blindness" and most people picture a world drained to grey. That is probably the single biggest myth about it. The overwhelming majority of people with a colour vision deficiency see colour perfectly well; certain shades simply collapse into one another, so where everyone else says "red" and "green" without thinking, they have to guess. A far more useful distinction is a different one. Seeing this way since birth is a trait: it stays the same for life and rarely gets in the way. Colours that have faded or shifted over the past few weeks are something else entirely. That is not a trait but a symptom, and it needs sorting out quickly.

Three types of cone, and what happens when one of them falls out of step

Colour is the work of the cones, light-sensitive cells packed most densely in the centre of the retina. There are three kinds, each tuned to its own slice of the spectrum, loosely labelled red, green and blue. The brain compares how strongly each kind responded and builds a shade out of the ratio between the three signals. Let one type respond weakly, or let its sensitivity drift towards its neighbour, and a whole band of shades stops being distinguishable: different colours start delivering the brain the same ratio.

That is where the varieties come from.

• Red-green deficiency is by far the commonest. Red, green, brown, orange and olive get mixed up, pink can look grey and purple can read as plain blue. The genes for the red and green cones sit on the X chromosome, which is why roughly one man in twelve is affected against roughly one woman in two hundred: a woman needs both copies to match before it shows.
• Blue-yellow deficiency is rare, inherited independently of sex, and equally common in men and women. Blue merges with green, and yellow with pink or pale grey.
• Complete absence of colour vision is a very rare condition, and it is obvious in infancy for reasons that have nothing to do with colour: the baby's eyes flicker, the child screws up their eyes and turns away from bright light, and fine detail is hard to see.

A woman carrying one altered copy of the gene sees colour normally herself but passes the trait to her sons. This is why colour blindness often appears to skip a generation within a family: the grandfather on the mother's side sees the same way, while the father has no trouble at all.

Traffic lights, sunburn and the child who never complains

An inherited deficiency causes no pain, does not blur the picture and does not reduce sharpness of vision. Plenty of adults find out about it by accident, at a medical assessment or after failing to read the number in a test they opened out of curiosity. The situations where it genuinely gets in the way are a fairly narrow set:

• reading a traffic signal, especially a distant one or one mounted horizontally, though the fixed position of the lights usually saves the day;
• judging whether berries and fruit are ripe, or whether meat is cooked through;
• spotting redness of the skin, whether a rash, sunburn or a wound turning inflamed. These can simply be missed, and this is the one thing in everyday life that is genuinely unsafe;
• coloured charts, diagrams, maps and highlighter pens, or the red and green indicator light on a device;
• putting an outfit together, which occasionally earns a raised eyebrow.

A child never complains about any of this, for a simple reason: they have no idea they see differently from everyone else. Suspicion usually arises between the ages of four and eight, and from indirect clues. Foliage comes out brown or pink in drawings, colouring in holds no appeal at all, sorting tasks by colour produce an unexpected freeze, colours are named hesitantly and with a glance at the next child's answer, and coloured chalk on the board draws the complaint that nothing can be seen. It is worth adding that marks can slip for reasons that have nothing to do with what the child knows, and nothing to do with attention span or ability either.

When the colours have shifted in the last few weeks

An inherited deficiency affects both eyes identically, stays unchanged across a lifetime and comes with nothing else attached. Anything that falls outside that description is acquired, which means it has a cause, and the cause usually matters more than the colour does. Reasons to take notice:

• the colours changed over days or weeks, having been perfectly distinguishable before;
• the two eyes differ: covering them in turn, red looks washed out and dull through one of them;
• sharpness of vision has dropped alongside the colour change, a patch has appeared in the centre of the field, or the whole picture has dimmed as though the brightness had been turned down;
• the eye hurts when it moves.

Call the emergency services, which across Europe means 112, or go straight to urgent care if vision in one eye has dropped sharply over minutes or hours; if severe eye pain comes together with redness, halos around lamps, nausea and vomiting; or if the change in colour perception arrived at the same moment as weakness in an arm or leg, a drooping face or slurred speech. Behind those combinations sit acute glaucoma, a blocked retinal vessel and stroke, conditions measured in hours.

Other things that shift colour perception in someone who used to see normally:

• inflammation of the optic nerve, more often in younger people, usually in one eye, with pain on moving the eye; sometimes it is the first sign of multiple sclerosis;
• glaucoma, diabetic damage to the retina, age-related macular degeneration;
• cataract: the clouding lens yellows and mutes the blues, which is why people are so often struck after surgery by how blue the world turns out to be;
• medicines. Ethambutol, used against tuberculosis, can damage the optic nerve, and losing the red-green distinction is sometimes the first sign, which is exactly why vision is checked on a fixed schedule during that treatment. Digitalis drugs, when blood levels run too high, cast a yellowish tint over everything and put halos around light sources. Hydroxychloroquine taken over many years calls for regular eye examinations;
• injury to the eye or head, a stroke in the occipital lobe, a tumour pressing on the optic nerve;
• vitamin B12 deficiency, and the combination of long-term alcohol misuse with smoking, both of which damage the optic nerve.

Doctors lean on one useful pattern: diseases of the retina tend to break the blue-yellow distinction, while diseases of the optic nerve tend to break the red-green one. So the question "which colours exactly have you started confusing?" is never asked as a formality.

Dotted plates, coloured chips and why online tests settle nothing

The first step is the plate test, where numbers or winding trails are assembled out of coloured dots. It is quick and catches red-green deficiency well, but it misses the blue-yellow kind and measures no degree of severity. Next come sets of coloured chips to be arranged in order of shade: the exact point where the sequence falls apart reveals which axis is affected and how badly. The most precise instrument is the anomaloscope, which identifies both the type and the severity, and its result is generally what is required wherever colour vision governs entry to a profession.

Tests on the internet are good for exactly one thing: prompting you to think and book an appointment. The screen, its brightness and its colour profile alter the image more than the eye itself does, and, crucially, no such test can separate an inherited deficiency from an acquired one, which is the question that actually matters. Where an acquired deficiency is suspected, an ophthalmologist will examine the back of the eye, check the visual field and, if needed, order imaging of the retina and optic nerve.

It makes sense to test a child before school starts, at around five or six, and without delay if someone in the family already has colour blindness.

Telling the school, choosing a career, and the workarounds that do the job

Inherited colour vision deficiency cannot be cured: there is nothing to put in place of missing or shifted cones, and the approaches written up as future treatments remain research for now. Acquired deficiency, on the other hand, is often reversible. Remove the cause and colour may come back, and the sooner the cause is found, the better the odds.

Filtered glasses and lenses deserve an honest word. They boost the contrast between the shades that get confused, and some people find them more comfortable; but they create no new cones, they do not restore colour vision, and no professional screening can be passed while wearing them. Anything advertised as glasses that cure colour blindness is marketing.

What genuinely works:

• telling the school. A teacher can back up colour with a written label, a shape or a hatching pattern, avoid building a task around shade discrimination alone, and stop putting anything important on the board in coloured chalk;
• finding out the colour vision requirements of a profession in advance. Pilots, train drivers, ship's officers, some military and police roles, electrical installation work and certain laboratory and medical jobs all impose them, but the standards differ from country to country and depend on the type and degree of the deficiency. This needs checking before applications go in and before a place of study is chosen, not at the medical;
• labelling anything distinguished only by colour: wires, paints, clothing tags;
• using the colour correction modes built into phones and computers, and apps that name the colour in a photograph;
• going by position rather than colour, whether that is a traffic signal, the order of buttons or the shape of an icon;
• asking for help wherever the cost of a mistake is high, such as judging whether the skin around a wound has turned red.

Online consultation

Two questions are well suited to being settled remotely. The first and most important is whether this is an inherited trait or a recent change. The doctor will work through whether it has been this way since childhood, whether both eyes behave the same, what exactly changed and when, what other conditions are present and what medicines are being taken, and will say whether an ophthalmologist needs to see you within days or whether it can wait. The second is living with colour blindness that is already known about: what to tell the school and how, which tests to expect at a medical assessment, how to factor the trait into a career choice, and whether the children should be checked. The warning signs listed above are not handled remotely; they call for urgent care in person.

This material is for information only and does not replace medical advice.