Every genre has a page, and the colour it is drawn in is voted on by the people who listen to it.
Every genre, descriptor and instrument has a page of its own: what it is, the records to hear first, what sits next to it, and what people here have filed under it.
Tags come from Last.fm listener tags and instruments from MusicBrainz credits. Both get filed onto a shelf the app keeps, so acid rock and heavy psych both lead to psychedelic rock instead of three thin pages.
Genres are drawn in colour all over the app: the bands of the river, the tiles of the map, the swatch beside a genre's name. That colour used to be worked out from the letters in the word, which was stable, fair and completely arbitrary.
Now it is voted on. Pick any colour you like from the swatch on a genre's page, not from a set of presets.
The largest group of people who broadly agree wins, and the colour is the average of that group only. A few votes for orange cannot drag a purple genre towards brown.