My feeling is that any visual effect should be used sparingly and appropriately. So if you want to colour a flashback, both to distinguish it and to set a mood/show how it is remembered by the protagonist, then fine. If you are adjusting colours for the whole thing, then be careful. Personally I'm OK with a muted colour palette in a noirish drama (classic or modern), but hyper-vivid colours drive me nuts in all contexts. I've never been able to see anyone would like falsely-saturated images like the early HDR produced.