Colour blindness checker
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Put in the colours you are using and this compares every pair of them the way three kinds of colour blindness would see them, then tells you which two stopped being two colours. Most tools stop at showing you a simulated picture and leave you to squint at small squares; the question you actually have is whether these two can be told apart, and that has an answer. Around one man in twelve has some form of colour vision deficiency, and the commonest chart palette in the world is a red and a green that mean opposite things.
The palette below is red, amber and green, which is the commonest one there is and the one that fails hardest. Change the colours and the answer follows as you type.
Your palette, as three kinds of colour vision see it
Typical colour vision
What most people see
Protanopia
Red looks dark. About 1 man in 100.
Deuteranopia
Red and green merge. About 1 man in 90.
Tritanopia
Blue and yellow merge. Very rare, and it affects women equally.
Pairs that collapse
Frequently asked questions
Are my colours sent anywhere?
No. Everything runs in this tab as you type. The site’s security policy sets connect-src ‘none’, so the browser blocks network requests from these pages, so nothing could be sent even if the code tried.
How does it decide whether two colours can be told apart?
By measuring how far apart the two colours are, simulating the deficiency, and measuring again. The measurement is a distance in CIE Lab, which is a way of describing colour built so that equal distances look equally different. Below about 10 the two are one colour. Between 10 and 20 they are the traffic-light case: not identical, but far too close to be the only thing telling two lines on a chart apart. Above 20 they are safely different.
Why does red and green survive protanopia here but not deuteranopia?
Because the two are not the same condition. A person with deuteranopia is missing the green-sensitive cone and sees red and green as one colour. A person with protanopia is missing the red-sensitive one, which carries most of red's apparent brightness, so red does not turn green for them, it turns dark. That leaves a lightness difference they can still use. It is a real distinction and this page reports it rather than flattening both into "red-green colour blind".
What is the fix when a pair collapses?
Vary lightness, not just hue. None of these deficiencies affects how light a colour is, so two colours that differ in lightness stay apart under all of them. A dark blue against a pale yellow works for everybody; a medium red against a medium green works for nobody. Beyond that, do not let colour be the only signal at all: add a shape, a label, a dash pattern, a direct annotation on the line.
Where do the simulations come from?
They are the matrices published by Machado, Oliveira and Fernandes in 2009, at full severity, applied in linear light rather than to the sRGB bytes directly. Using the published set is deliberate: somebody who checks a palette here and again somewhere else should not be given two different answers.
Is a simulation the same as seeing it?
No, and it is worth being honest about that. These matrices model dichromacy, where one type of cone is absent. Most colour vision deficiency is anomalous rather than absent, a shifted sensitivity rather than a missing one, and it comes in every degree. Full dichromacy is the hardest case, so a palette that survives it survives the milder ones, which is why checking against it is useful even though it is not what most affected people see.
How many people does this affect?
Roughly one man in twelve and one woman in two hundred, for the red-green kinds, which are carried on the X chromosome. Tritanopia is far rarer and affects both equally. On a page with a thousand visitors that is dozens of people who cannot read a red-versus-green chart, and none of them will write in to say so.