I have just one thing to add to Meredith's great answer. Gatsby's shiny color preferences remind me of my own birds, who like to pile of shiny things up just for the sake of having them. They don't really do anything with the shiny toys, just display them, shake them about a bit. Isn't this what Daisy is doing with Gatsby and vice-versa? And isn't Daisy sometimes exhibiting the depth of a piece of tin foil by her willingness to become an object in the cache? I would also argue that Jordan and Myrtle want to be the shiny objects as well. Only Nick releases (eventually) the essential worthlessness of all the pretty toys.
Along with Meredith's sensible and true "gray" comments about Gatsby's suit, I would also argue that the neither-black-nor-white of "gray" symbolizes Gatsby's own caginess, his attempts, like the dirt, to hide what he truly is.
As to the white shirt, again, it is true in a literal sense that white was difficult to keep clean, but I don't think Gatsby nor his entourage cared one wit about old money. They all fail to realize the dignity that comes along with practiced manners and subtlety. Those in attentdance at Gatsbys fetes are like moths to his flame, short lived and short sighted.
The effect is that he attracts notice.
Gatsby's clothing can be read as symbol for his new wealth. White is sometimes the symbol for newness and indicates that he is in no danger of getting his suit dirty and that he can afford to wear white. He was living in a time where laundry was a more difficult task and it was hard to keep white items clean. So his suit reflects this. The silver shirt and the gold tie also represent wealth - namely gold and silver.
It is a very audacious outfit to have on and one that would have attracted scorn from the old money and notice from everyone else.