Approximately 8% of men and 0.5% of women have some form of color vision deficiency (CVD). The most common types, protanopia and deuteranopia, reduce the ability to distinguish red from green.
The Red/Green Problem
The classic example: red for error, green for success. For a user with red-green CVD, these look nearly identical. The fix is not to avoid red and green. It is to never rely on color alone to convey meaning. Always pair color with a secondary indicator: an icon, text, shape, or position.
Building CVD-Friendly Palettes
You do not need to avoid any specific colors. You need to ensure that every color pairing in your system differs in lightness, not just hue. Two colors that are the same lightness but different hues are the hardest for CVD users to distinguish. Two colors with clearly different lightness are distinguishable even in grayscale.
This aligns naturally with WCAG contrast ratios. A 4.5:1 contrast ratio inherently means a significant lightness difference. So meeting contrast requirements gets you most of the way to CVD accessibility for free.
Testing Your Palette
The best way to test is to view your palette in simulated CVD modes. In PaletteRx, the contrast ratio testing in Step 3 gives you objective numbers. If two colors pass AA contrast, they are distinguishable by lightness even for CVD users.