Use of Color
Color must not be the only visual means of conveying information. Add icons, patterns, underlines, or text labels alongside color coding. Affects 300M people with color vision deficiency.
Never let color be the only clue. If red means 'error', add a word or icon as well — otherwise color-blind users miss the message.
Why this matters
Around 300 million people have color vision deficiency. If 'red means error, green means OK' is the only cue, they can't tell the states apart — and neither can anyone on a grayscale printout or a failing screen.
How to detect
View the page in grayscale (Chrome DevTools → Rendering → Emulate vision deficiencies). Can you still tell links from text, errors from success, required fields from optional? If meaning disappears, add an icon, label, or pattern.