What colour blindness is, and what it is not

The moment you find out

Often it is small things. Your child colours the grass brown and the teacher mentions it. A friend asks why you wore odd socks. A charger light that everyone else calls green looks orange to you. Or a work medical hands you a sheet of coloured dots and you cannot see the number.

Colour vision deficiency is the proper name for what most people call colour blindness. The National Eye Institute (NEI) describes it as seeing colours differently from most people, usually with trouble telling certain colours apart. Your sight is otherwise normal. Seeing no colour at all, only shades of grey, is rare and is a different condition.

Most people with the common inherited type have had it since birth and never knew anything was unusual. The NEI notes that many cases are mild enough that people do not realise, and that most adjust to it. Our page on colour vision explains the assessment we offer; this guide covers the basics first.

The types, in plain words

The retina at the back of your eye has three kinds of cone cells, tuned to red, green and blue light. When one kind is missing or responds to a different band of light than usual, the brain gets a muddled signal and two colours can look the same. The American Academy of Ophthalmology (AAO) explains that the degree varies: some people only struggle in dim light, others cannot separate certain colours in any light.

The NEI groups the types this way:

• Red-green, the most common group. Deuteranomaly (certain greens look redder) is the most common and usually mild. Protanomaly makes certain reds look greener and less bright. In protanopia and deuteranopia, red and green cannot be told apart at all.
• Blue-yellow, a less common group. Blue and green, or yellow and red, are hard to separate, and in tritanopia colours also look less bright.
• Complete colour vision deficiency, which is rare. No colour is seen, and depending on the type a person may also have poor sharpness of vision and be sensitive to light.

Why it runs in families and affects more men

The NEI puts it at about 1 in 12 men. Women are affected far less often. The reason is in the genes: the NEI explains that red-green deficiency is passed down on the X chromosome. A boy has one X chromosome, from his mother, so if it carries the gene he has the condition. A girl has two, and both would need to carry it, which the NEI describes as rare.

A woman who carries the gene can pass it to her sons without having the condition herself. That is why it often skips a generation and surprises parents. Blue-yellow and complete deficiency involve other chromosomes and, according to the NEI, affect people of different sexes equally.

Colour vision can also change later in life. The NEI and the AAO list eye disease such as glaucoma and macular degeneration, cataract, diabetes, multiple sclerosis, Alzheimer's disease, some medicines such as hydroxychloroquine, and injury to the retina, optic nerve or brain. The AAO notes that acquired colour loss often affects the two eyes differently and can get worse, whereas the inherited kind stays the same for life. Our post on the types and causes of colour deficiency goes into more detail.

How it is tested

The test most people meet is a set of plates made of coloured dots with a number or shape hidden inside. The NHS names this as the Ishihara test, and also describes arrangement tests, where you put coloured caps in order by shade. The Farnsworth D15 is one of those, and it is the follow-up test named in both the NZ Police and Civil Aviation Authority standards described below.

In New Zealand a colour vision check is part of a standard eye examination, according to Healthify NZ. A screening tells you whether something is there. A fuller colour vision assessment works out the type and how marked it is, and gives you a written report, which matters if an employer or regulator has asked for one. Our consultation fees page lists what each appointment includes.

For children, the NEI suggests testing if there is a family history or the child has trouble learning colours. There is no reading involved; a child only needs to trace a shape or name a number. Online tests exist, but the NHS advises getting checked properly if you have any concern. Our post on colour blindness testing describes what the appointment looks like.

Work, study and licences in New Zealand

Some occupations test colour vision, and each sets its own standard. Where we could open the official page, this is what it says. For anything else, check the employer's or regulator's current standard rather than relying on a general rule.

NZ Police. The Police medical standards for recruits (November 2021) screen with the 24-plate Ishihara test and move to the Farnsworth D15 if three or more errors are made on plates 2 to 17. The standard is not met if both tests are failed or the applicant has a red (protan) deficit, and the Police overview lists colour vision deficiencies that affect the role among the conditions it treats as incompatible.

Pilots. The Civil Aviation Authority's colour vision guideline requires no colour vision deficit of aeromedical significance. Passing Ishihara meets the standard. If you fail it, further tests such as a lantern test, the Farnsworth D15 or the CAD test can still give an unrestricted certificate, and a practical flight assessment can give a certificate with restrictions, such as no night flying.

Defence Force. Defence Careers says each trade sets its own level of colour perception, some trades accept a reduced level, and candidates should discuss options with their Candidate Engagement Facilitator. Trades such as electrical work may also test; we could not open an official standard for them, so ask the employer. For driver licensing, NZTA (Waka Kotahi) sets the eyesight requirement; check its current page rather than this guide.

Tinted lenses are not a route through any of these tests. Our guide to EnChroma and ChromaGen covers what they do and do not do. At school, the NHS suggests telling the teacher so lessons and exams can be adjusted, for example by labelling colour-coded charts.

When this practice is the wrong place

Inherited colour vision deficiency is stable and never an emergency. A change is different. The AAO advises that anyone who notices a significant change in how they see colour should be assessed, because it can be a sign of a condition affecting the retina, optic nerve or brain. If colour has recently gone dull or washed out, especially in one eye, book an eye examination promptly or see your GP, and mention any medicines you take.

Call 111 or go to the nearest hospital emergency department, rather than booking with us, for sudden loss of vision, a shadow or curtain across your vision, flashes with a sudden shower of new floaters, a chemical splash, a red and painful eye with light sensitivity, or an eye injury. Healthify NZ gives the same advice for sudden severe eye pain or vision loss.

What happens next

If you simply want to know, a comprehensive eye examination includes a colour vision check alongside everything else; our guide on what an eye exam checks walks through it. If you need a report for an employer, or want to understand the type and degree, book the colour vision assessment described on our colour vision page. No referral is needed in New Zealand.

You may not need to buy anything. Most people manage well with habits: labelling clothes, learning the position of traffic lights, and using a phone app that names the colour you tap, which the NEI lists as a useful aid. If you are curious about tinted lenses, read the comparison guide first so you know what to expect before you spend.