From reddit: "Foncé" is only used for colours, while "sombre" works in any context (including colours, so "une couleur foncée" or "une couleur sombre" mean almost the same. However, if you speak of a specific colour, you must use "foncé", so "du bleu foncé").
"Sombre" can also be used for :
"not enough light", for example "Lorsque le soleil se couche, il commence à faire sombre."
in the figurative sense, it means something like "sad and grave". "Le 11 septembre est un jour sombre dans l'Histoire des États-Unis."
From what I've read I've put this together which may help you.
Sombre and foncé are interchangeable when describing the shade of dark. Vous avez un costume sombre – You have a dark costume Or Vous avez un costume foncé – You have a dark costume would both be correct and would both relate to the shade of colour.
If we accept this rule then la bière est sombre should be accepted, however any french speakers that know of any idiomatic or cultural reasons why beer in particular might be called foncée over sombre please jump right in and correct me!
In the example of another thread marron foncé is made from two adjectives. In this instance Les chaussures sont marron foncé is correct But Les chaussures sont marron sombre would be incorrect
This is because the colour (main adjective) has been modified and this can only be modified with foncé
Sombre can be used figuratively as I’m sure you’ve read many times.
Perhaps think of nouns and adjectives. Remember sombre shouldn’t be used to modify colour adjectives Sombre can be used to describe a noun (including a colour) Les couleurs sont sombres is correct because ‘the colours’ constitute the noun.
In conclusion Try to separate the nouns from the adjectives Ask yourself ‘Has a colour adjective been modified?’
With this information you can decide whether sombre is totally inappropriate to use.
Some respected users say that foncé is more commonly used to describe the colour of a thing, but then other moderators, seem to think that sombre and foncé are interchangeable in describing the colour of a thing.
I think it boils down to nouns, adjectives and whether or not you are modifying a colour adjective.
"Sombre" is used to express the idea of lacking in light. Consider these examples of "sombre" used in context: http://context.reverso.net/traduction/francais-anglais/sombre
If one is talking about a color being of a dark shade, one would generally use "foncé", but one may also refer to the color of an object as being "sombre" (FR).
Sorry ripcurlgirl, a little more clarification please :)
Are we saying here that the safest thing to do would be to use foncé to describe colours and to use sombre figuratively?
I have seen comments which say that sombre can be used to describe colour, by moderators like sitesurf.
'"Foncé" and "sombre" can be synonymous when it comes to describe colors (dark). However, "sombre" has other meanings, especially figurative, that "foncé" does not have.'
I understand that sombre can be used figuratively, but i'd like to know when it can be used to describe the darkness of colour. Could you possibly give us a few examples of sombre in this context?
There are so many comments that seem to (almost) answer this, but I think we'd all benefit from tangible examples to tell us what is right and what is not. If duolingo needs to update the lesson, then at least us learners are aware that it's being looked into!
Zakir's reply throws some light on the subject, but still from the discussions so far, I also could not make out a clear difference between "sombre" and "foncé". If 'Sombre' is for 'sad & gloomy' state, then why use 'sombre' in sentences like "Les chaussures sont sombres" ?