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Short answer
About 8% of men and 0.5% of women have some form of color blindness. Relying on color alone can make it impossible to distinguish system states such as success and failure.
Explanation
Common types include red-green color blindness (Deuteranopia and Protanopia) and blue-yellow color blindness (Tritanopia). The design solution is "color independence": always pair color with a secondary visual indicator, such as a checkmark (✓) with green, an exclamation mark (!) with red, and an underline for text links so they differ from ordinary text.
Common mistakes
- Giving invalid input fields only a red border without a warning icon or error text explaining the problem.
- Designing pie charts that rely on similar shades of green and red to distinguish gains and losses.