Green Light: A green light into the future. A hope or longing Gatsby is searching for. Gatsby is thinking of Daisy as the light in the darkness. Nick also refers to the light as America rising out of the water as a beacon for foreigners. Since this is strongly representative of the American Dream, the color green could also represent money and prosperity.

Valley of Ashes: Represents the moral decay of America, due to materialism and wealth, with the dark ashes. With the valley being close to where Gatsby and Nick live, it symbolizes a crash coming near, with few people noticing or realizing it even if they "drive through." 

Doctor T. J. Eckleburg's owl eyes: The fading eyes painted on a billboard in the valley of ashes can represent God watching over the morally in just  people. Since few people pay attention to the eyes, it could be saying that few people pay attention to God and don't care that they are morally in just. These eyes also show up on the man in the library.

Symbolism