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Brewer palettes (http://mkweb.bcgsc.ca/brewer/) are hand-picked colour ranges that were originally meant for cartography but are useful for general data viz, where it is important to maintain perceptual regularity while displaying quantitative data.

There are Brewer palettes geared for sequential data, where you further need to express proportionality along a scale; and "diverging" data that has a natural zero point with two extremes where you need to present a spectrum. In all these cases it's important that one category doesn't appear "heavier" than another, and that subjective notions like "about twice as intense" reflect the underlying data.