I haven't read a lot of the thread, but I suspect that the lack of color in HD games comes from developers trying to produce games that look more "Realistic" and are "Gritty" ... Many of these games are probably drawing a lot of inspiration from movies like Aliens which (generally speaking) are very dark and don't offer much color.
Personally, if I was creating a game with these city scapes that had been destroyed generations before (like so many games seem to have) there would be lots of plant life taking over the city (including flowers of various colors) because it works as a decent metaphore, and creates an interesting juxtaposition.







