Color blindness (more precisely, color vision deficiency) is when someone has trouble distinguishing some colors — often red and green, but other combinations too. It affects a large share of the population, mostly men, and it is usually lifelong.
On the web, that means a red “error” border with no text fails, a green “available” dot with no label fails, and a CTA that differs only by hue from the background may disappear. Pair color with words, icons, or patterns. Strong contrast between text and background helps everyone, not only people with color vision differences.
A real-life example of color blindness
A form marks invalid fields in red only. A user who can’t tell red from green sees no difference from valid fields — and submits anyway. Adding “Required” text and an icon fixes it for them and for screen-reader users too. That’s core accessibility and WCAG thinking.
Test with contrast checkers and don’t use color as the only signal. See How to Design an Accessible Website and Web Accessibility and Inclusive Design.