Never rely on color alone

Why this rule exists

Color is a wonderful reinforcement and a terrible sole signal. A meaningful fraction of people cannot reliably distinguish certain hues, and every user meets moments where color fails, on a monochrome print, a failing screen, a glance too quick to register a subtle tint. If the only difference between a valid field and an error is red versus green, or the only marker of the selected item is a colored background, you have built information some people simply cannot receive. The fix is not to abandon color but to pair it with a second cue: an icon, a label, a shape, a change in weight or position. Color then becomes the fast, pleasant reinforcement it is best at, while the redundant cue guarantees the meaning survives when color does not. Designing this way costs little and quietly includes a great many people who would otherwise be left guessing.