Gray
Gray is a representation of despair and sadness. In the Valley of Ashes, almost everything is gray. The city is full of the poor class society who have no chance at ever getting out of there.
If you see gray, steer clear--you don’t want to associate yourself with any of those low-class peasants. If you need an example, just take a look at Wilson--he had an “ashen” face, and he ended up going crazy and shooting someone and then himself. You don’t want that to be you; stay away from the gray.