Short Guide to Accessibility by an Actually Colourblind Person v1.0

These things are wrong, misleading, or potentially misleading. Do not use them to validate your colour schemes:

- Web Content Accessibility Guidelines (WCAG)

- Colour blindness simulators

- Advice from designers with full colour vision

- Your ideas about how colour-blind people see colours

One myth is that most colourblind people don't "see" a colour, or a difference between two colours. Seeing is a complex sensory and cognitive process, involving the brain filling in the gaps. Colourblind people simply have less raw sensory data coming in to work with.

For example, we may see a difference between two colours but only if they're next to each other, or if they're on big enough shapes. The true test is whether we can navigate your designs without squinting, staring, or unnecessary cognitive load.

If possible, do not rely on colour to convey anything meaningful. If not possible, make an alternative *high-contrast* version. Use either grayscale or a simple low-saturation colour ramp. Make it good enough that *you* wouldn't mind using it yourself.

Don't use the existence of a grayscale version as an excuse to not make your colourful version as accessible as it can be. If you prefer your colourful maps and diagrams to monochrome ones, most of us will too - if we can use them.

Distinguish colours primarily by brightness, and secondarily by hue. Use HSV, not RGB, to reason about colours.

If two colours have a significantly different brightness, they are fine to use together.

If two colours have a similar brightness, fully saturated colours should be at least 60° apart (hue). The more the merrier. Medium saturated colours should be even further apart hue-wise. Low saturated colours should not be used together, or with grayscale colours of similar value, at all.

Fully saturated colours require larger differences in brightness to be distinguishable than less saturated colours, because people may or may not perceive a colour as darker depending on the type of colourblindness.

Please remember that the above rules of thumb are no substitute for a test involving people with different kinds of colourblindness.

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1. credit is given to the author

2. whole text is included unedited, including this notice

-- kiilas (@kiilas@computerfairi.es)