I have a branch in a specific colour, with its first checkin having the color set and the 'propagate to descendants' enabled.. Then I change one of the checkin colours of the middle to a specific colour, and all next checkins get 'color none'.

I'd expect that the original color set to propagate is what goes to the next checkins of the branch.

In fossil-scm, there is some approach like that in trunk for the releases, but making such coloured 'released' checkins from a coloured non-trunk would not work that easy, in the current fossil version.