One of the central aspects of this story that is very worthy of closer analysis is the way in which Fitzgerald uses colour symbolically. For the purposes of this question, the colour green is very important. Throughout the novel, the colour green is used to symbolically represent the hopes and dreams of Gatsby and, more generally, the American Dream.
The most obvious reference to the colour green is the light at the end of the dock that is opposite Gatsby's house that Nick sees Gatbsy so focussed upon at the end of the first chapter, stretching his arms towards it:
But I didn't call to him, for he gave a sudden intimation that he was content to be alone--he stretched out his arms toward the dark water in a curious way, and, far as I was from him, I could have sworn he was trembling. Involuntarily I glanced seaward--and distinguished nothing except a single green light, minute and far way, that might have been the end of the dock.
It is only later that Nick discovers this green light indicates the house of Daisy Buchanan, the focus of all of Gatbsy's hopes and energies. It is particularly relevant then to see how at the end of the tale Fitzgerald links in this green light with the "fresh, green breast of the new world" that the original Dutch sailors saw as they arrived at what was to them a new Garden of Eden. This "green breast" finds a pale comparison in the artificial green light of Gatsby's imaginings, suggesting how the original American Dream has been lessened or transformed into something very different from what it once was. We are told that "Gatsby believed in the green light," but the novel clearly shows where believing in the green light will take you. The American Dream only ended in tragedy for Gatsby, and Nick's departure from the East back to the Mid-West suggests his recognition of the dangers of pursuing such an "orgiastic future."