Sequential palette

A sequential palette is a quantity written as colour. One hue is sampled at a series of steps that get steadily darker, the data is mapped onto those steps, and the reader decodes the picture by asking which cells are darker than which. The whole thing rests on lightness being the one attribute of colour the eye ranks without being taught: nobody has to be told that the deep end means more, because dark reads as heavier than pale before the legend is consulted. Hue and colourfulness may move along with it, and in the better ramps they do, but only ever in support of a lightness curve that never turns back on itself.

That is exactly the property a rainbow ramp lacks. Red through yellow through green through blue has no ranking anyone shares, and its lightness wanders, so a bright yellow band appears in the middle of smooth data and reads as a boundary the data does not have. The modern replacements (viridis, magma, and the single-hue ramps in most design systems) were built by walking a perceptually even space so that equal steps in the number are equal steps to the eye, which also means the picture survives being printed in grey or read by someone with a colour vision deficiency, since the order was never carried by hue in the first place.

In practice the decisions are how many steps and where they fall. Five to seven classes is the usual ceiling for a map or a heat grid, because a reader matching a cell back to a legend cannot reliably tell apart more than that, while a continuous ramp is fine when the job is spotting shape rather than reading values. Keep the pale end clearly distinct from whatever “no data” looks like, since an empty cell and a nearly zero one are different claims, and remember that any label sitting on the dark end needs its own contrast check.

The two neighbouring shapes answer different questions. A diverging palette is two of these ramps back to back around a meaningful middle, for values that run above and below something real. A categorical palette abandons order entirely and spends distinct hues on groups that have no rank, which is what makes lightness free to be even across all of them.