Color Scale

A color scale is the rule that maps numbers to colors. There are three kinds for three jobs: a sequential scale (light to dark of one hue) for data that runs low to high like population density; a diverging scale (two contrasting hues with a neutral middle) for data with a meaningful center like temperature above or below average; and a qualitative scale (distinct unrelated hues) for unordered categories like country names.

Choosing the wrong type quietly distorts the message — a diverging scale on plain low-to-high data invents a center that does not exist. A second pitfall is the old rainbow scale: it looks vivid but its brightness jumps unevenly, creating false boundaries, so perceptually uniform scales such as viridis are now preferred for honesty and color-blind safety.