Colour

Meaningful, not decorative.

Three palette types

Categorical for unordered groups, sequential for a quantity, diverging for values above and below a meaningful midpoint. Using the wrong type invents structure.

Rainbow scales create false boundaries and are not perceptually even.
Limit categorical colours to about seven; beyond that, group or label directly.

Accessibility

Around one in twelve men has some colour vision deficiency. Encode meaning by position, shape or direct labels too, and check contrast for text on coloured backgrounds.