Taken from the W3C

Ensure that text and graphics are understandable when viewed without colour.

If colour alone is used to convey information, people who cannot differentiate between certain colours and users with devices that have non-colour or non-visual displays will not receive the information. When foreground and background colours are too close to the same hue, they may not provide sufficient contrast when viewed using monochrome displays or by people with different types of colour deficits.

Details:

1. Colour blindness - colour wheel.
2. Colour blindness - photo example.
3. Age effects.
4. Without colour.
5. Dyslexia.
6. Flashing, flickering or blinking content.
7. Summary.

Who does this affect?

• Users with monochromatic displays.
• Users with vision impairments using screen reading or braille technologies.
• Text only browsers.
• Those with photosensitive disorders (migraines, epilepsy) are affected by flashing.
• People aged forty plus are affected dy contrast.
• People with colour perception difficulties ().
• People with dyslexia are affected by colour choice.

The WAI guidelines:

• 2.1 Ensure that all information conveyed with colour is also available without colour, for example from context or markup. [NGfL requirement]
• 2.2 Ensure that foreground and background colour combinations provide sufficient contrast when viewed by someone having colour deficits or when viewed on a black and white screen. [Priority 2 for images, Priority 3 for text]. [NGfL warning]
• 7.1 Until user agents allow users to control flickering, avoid causing the screen to flicker.
  Note. People with photosensitive epilepsy can have seizures triggered by flickering or flashing in the 4 to 59 flashes per second (Hertz) range with a peak sensitivity at 20 flashes per second as well as quick changes from dark to light (like strobe lights). [NGfL requirement]
• 7.2 Until user agents allow users to control blinking, avoid causing content to blink (i.e., change presentation at a regular rate, such as turning on and off). [NGfL warning]

Further information:

• Juicy Studio: Don't rely on colour alone
• Diseases and conditions simulator