Make color mean something

Why this rule exists

In a well-designed interface, color is a language, and every hue the user learns should keep its meaning wherever it appears. When your primary blue means the main action in one place and a passive link in another and a decorative flourish in a third, you have taught the user nothing they can rely on, and color stops informing and starts decorating. Consistent, meaningful color is one of the quiet accelerators of usability: once a user learns that a certain color means go, or danger, or interactive, they read the interface faster everywhere. Color used purely for looks dilutes that vocabulary, because every non-meaningful use makes the meaningful ones a little harder to trust. Decide what each color says and then keep the promise, so that seeing the color is enough to know the meaning without reading a word.