"Vous avez un costume sombre."
Translation:You have a dark suit.
- foncé = dark in colour
- sombre = dark in colour or dark in mood (a funeral suit, perhaps)
- grim = dark, but I would apply it to people or situations ("He had a grim look on his face" "The situation is grim, the war is going badly").
If I said (UK English) "That's a grim suit" I would be being sarcastic. It would suggest the suit was unfashionable, badly made, garish, or in some other way horrible or unsuitable.
I think it's because "somber" in English is almost exclusively used to refer not only to a color, but the associated feelings it invokes. For example, "dark-colored" is a very neutral description, but "somber color" makes me think of something serious or even sad. I'm not sure if the same subtlety applies in French.
(I am not native, but a native has explained this to me)
Sombre can mean dark or sad/grim or both; similarly, the word dark can mean of dark color or grim in English. However, in this situation, it means of dark color. Technically, you should be able to say "foncé," because it deals with color, but I have not tried on Duolingo.