Those colours are changing or disappearing completely?
At first I thought it was my imagination, but twice in the past month I’ve marked mails with color categories, and the next day the markers were not there.
does this happen to all your emails or only to some accounts? Some servers don’t support color categories so eM Client holds this information locally, but when some attributes of the emails change on the server side, it writes over the setting in eM Client. Sadly, we can’t solve this if that is the issue.