When suntanning in a portable recliner in the daytime the tan itself can actually outlast the moodlet seemingly becoming permanent. I've not had them clear, but of the possible causes it may also be related that I went to the Spa shortly after tanning to get a pedi/mani, which may have thrown the timers into disharmony (Just speculation).
In any case the sim in question SHOULD only have in-game acquired skin details, so the tan is the only thing on them. When I try to change their skin color through create-a-sim they remain tanned as if it is glitched.
Edit By Crinrict: Added missing pack tag to title.
