I feel like this has nice overlap with Ability to name patterns and Possibility to have more and more accessible color schemes .
Keeping in mind that the feature is more important than the realization, I’d be okay with having to edit a text file on the SD card (JSON or key:value) to modify the list of colors, similar to the “color schemes” proposal. No need for a complex color editor on the device. I guess specifying RGB wouldn’t be too hard with the physical interface, but choosing from a preset list would be even easier. Expressing names as typical CSS colors might be a nice-to-have. But just RGB seems like enough.
The immediate recognition of what patterns are used where is what reminds me of the naming suggestion.
In keeping with tracker tradition, ideally colors used in a song would travel along with that song when shared with others.
There are some edge cases that people could run into, like accidentally ending up with black on black. Guidance for how to reset to defaults (e.g., “just delete this text file from the SD card”) seems like it would be sufficient to deal with this.