Acide vs. Sur vs. Aigre
I have seen these all meaning something like "sour," but what are the differences and are they subtle?
Have you tried wordreference? It provides more detail than many dictionaries. My guess is that it depends on the context and that the differences are similar to those between acidic and sour (and maybe even bitter). No doubt someone else can provide a better answer but I thought this was an interesting question.
sur - sour, bitter http://www.wordreference.com/fren/aigre
Acide and aigre seem to have many more meanings related to this and can be used metaphorically: