category-extra

This worked brilliantly for colour-coding the categories, which is what I wanted. Unfortunately it made a weird salad of my taxonomy: child categories attached themselves to different parents, it wiped out my category descriptions and stuck a whole bunch of unwanted "meta" boxes above the actual SEO fields. It looks like something the author wrote for a particular purpose or website; perhaps they should have tested it in a wider variety of environments. Since activating this plugin, I've been getting a debug warning that has something to do with taxonomies. I'm off to repair everything - and hand-code my category colours. If nothing else, the Category Extra showed me this can be done fairly simply, so I'm grateful for that.