a setting in the sequence/sound properties dialogs would be fine IMO
perhaps a user could first set the track colour in the track header (the way we do now) and all new sequences/sound clips would be coloured this way automatically. But, then the user can later change individual sequence/clip colours if they wish – which is what I would do.
Changing the track colour header again would probably overwrite all the sequence/sound colours across the whole track, logically, but the user can “undo” if they did this by mistake