Pick a hue and commit

A working palette is smaller than people expect. One accent hue with a light and dark variant, a neutral ramp of eight to ten steps, and semantic colours for success, warning and danger. Everything on the screen comes from that, and the discipline of having nowhere else to go is what produces coherence.

The five-swatch palettes that colour tools generate fail because they answer a different question. They give you five equally weighted hues, and an interface does not want five equally weighted anything: it wants a lot of near-neutral, a little accent, and clear semantic exceptions.

Decide what the accent is for before you pick it. If it means the primary action, then it appears on primary actions and nowhere else, and a page with nine accent-coloured elements has told the reader that none of them matter. Restraint here is not aesthetic preference, it is how the colour keeps meaning anything.