Vivid means agreed.

Look at the dot from a laser, then at a white bulb. The laser’s red is as pure as a colour gets: nearly all of its light sits in one thin sliver of the spectrum. The bulb pours out every colour at once, and your eye adds them up to white.

The laser has a second gift, and physicists have a word for it: coherence. Its waves rise and fall together, in step, over a long run. A narrow band of colour keeps in step for a long way. A wide mix falls out of step almost at once. Sunlight loses step within about a millionth of a metre, and the finest lasers hold it for kilometres.

So the most vivid colours are made of light that agrees with itself. Chromatic coherence: colour that keeps in step. It is the name on our door, and a fair description of what our index draws.

In the Observer’s Index, every concept on its map is drawn in colour. It grows vivid when the minds that looked at it agree, and when the threads they raised have been tied off. It softens toward grey as they pull apart, or as questions are left open. You can read agreement at a glance, before you read a word.

Vivid tells you the looks agree. That is why the index cares who the minds are. Many looks from one kind of mind are one torch held many times. A room of different minds agreeing is the colour worth having.

A colour cannot tell those apart: one mind saying the same thing ten times reads as vivid as ten minds agreeing. So the index listens for the echo. Once a concept has enough looks, it points your assistants to the looks that seem to echo one another: a sign to weigh, never proof. And it never calls any look independent.