Color coding

Colour coding is a substitution. A category stops being a word and becomes a hue, and the reader is asked to learn the mapping once so they can read it everywhere afterwards. What that buys is the one question colour answers faster than text: grouping, and finding. Hunting a week of calendar entries for the interviews, or a log for the network lines, is close to effortless when the target has a hue of its own, and it means reading every label when it does not. The cost is the learning, which is why a distinction that appears once is not worth coding: until the mapping is known, a coloured mark carries less than the written word it replaced.

The set size is where colour coding usually fails, and the ceiling is lower than people expect. Christopher Healey’s survey of perception research puts the reliable number of distinguishable colours somewhere between six and twelve, and that is for colours seen in isolation, not small marks scattered across a busy screen. Past the ceiling the hues have to sit close enough together that two of them read as one colour seen twice, and the reader stops matching marks back to the key at all. A code set that has outgrown the limit is not repaired by picking better colours. It is repaired by coding fewer things: merge the rare categories, code only the few the screen is actually about, and let the rest be plain.

Four neighbours own the parts of this that are not the coding itself, and they are easy to blur together. A categorical palette is the set of hues, chosen so that none of them outranks another; colour coding is what happens once that set is pointed at real categories. Status colour is a coding whose categories are fixed outcomes and whose mapping most readers already hold, so it carries no learning cost and never grows past four. A chart legend is the written half of one chart’s mapping, the key rather than the code. And use of colour is the constraint over all of them, the rule about what a coding may not be the only carrier of.

That constraint is also the practical advice, because a coding is at its best when it is never alone. A shape, a position, an icon, or the label the mark was going to carry anyway turns the hue into an accelerator rather than the sole channel, and the mark stays readable for a reader with a colour vision deficiency, on a projector, or in a screenshot printed in grey. Syntax highlighting is the colour coding most people look at all day, and it is the clearest demonstration of the whole idea: a small closed set of token categories, one colour each, held identical across every file, with weight and italics carrying the same distinctions a second time.