/01 Seeing your design as others see it
Load an image or a palette and preview how it reads to the common forms of colour vision deficiency — protanopia, deuteranopia and tritanopia. Around one in twelve men and one in two hundred women has some form of it, which is a substantial share of any audience.
Simulation makes the problem visible in a way that guessing does not.
/02 Designing so colour is never the only signal
Red and green are the pairing most often confused, which is unfortunate given how universally they encode success and failure. A red error and a green success message can look identical, and a chart distinguishing its lines by hue alone becomes unreadable.
The fix is redundancy: pair colour with a shape, an icon, a label or a pattern, and keep sufficient lightness difference between elements that must be told apart. Something distinguishable in greyscale will generally survive any form of colour vision deficiency.
/03 A color blindness simulator for real design work
Upload a screenshot of an interface, a chart, a map or a photo, or pick a single colour, and see it as people with eight types of colour vision deficiency do. The red-weak types are protanopia and the milder protanomaly. The green-weak types are deuteranopia and deuteranomaly. Tritanopia and tritanomaly affect blue and yellow, and achromatopsia and achromatomaly remove colour almost or entirely.
Red-green colour blindness, the protan and deutan types, is by far the most common, affecting roughly 8 percent of men and 0.5 percent of women of Northern European descent, with deuteranomaly the most common single type. Start there when checking a design. Look at anything that relies on red versus green: error and success states, stock and status indicators, chart series, map legends and required form fields.
/04 A simulator, not a vision test
Seeing an image through a simulation shows designers what colour-blind users experience; it cannot tell you whether you are colour blind. Screen colours vary between devices, and anomalous types differ widely in severity from person to person, so a simulation shows a typical case rather than every case.
Online colour blindness tests viewed on a normal screen share the same weakness, since they cannot control the colours shown. An optometrist can screen properly using Ishihara plates or an anomaloscope under controlled conditions.