I don't think the resale value will be that dependent on the color. Any item is worth what some one is willing to pay.
For example, if I am trying to sell a pink 5c to some one who is buying it for some one who hates pink, it will likely not be worth anything to them. If some one else is buying for a person whose favorite color is pink and they have had a hard time finding a pink 5c, my item become much more valuable to them. Supply and demand.
I would guess that trade in values at places like Best Buy or any of the carriers will be the same no matter what the color is. Of course, that is pure speculation.