Color symbolism is exteremely important in this novel. I strongly urge anyone to consult the University of Michigan's Online Dictionary of Symbolism and look up the various colors Fitzgerald uses, namely yellow, green, white, blue, gold, and silver. Remember that any given color may have multiple suggestive meanings depending on culture and context. For example, white usually suggests innocence and purity. White is clearly associated with Daisy, but she is hardly innocent. Instead, white for Daisy suggests transparency. She is so shallow that we can see right through her. In the case of the car, the yellow exterior suggests decay and corruption [note the rims of T.J. Eckleberg's glasses are also yellow]. The green interior suggests money (of course--it is a Rolls Royce!)as well as hope and innocence. In combining these two colors, Fitzgerald is suggesting that hope and innocence (associated with the West in TGG) are often corrupted by greed and loss of hope. The car, then, is sort of objective correllative for Gatsby himself. Invariably, I get at least one student who questions how much of these things an author "intends." Fair enough. I usually answer by saying that writers have choices. He could have made it any color he wanted, but he chose these for very specific reasons. If the color symbolism conveniently reflects the themes in the novel--so be it.
The color of Gatsby car is important as this is the vehicle that ultimately kills Myrtle, The car itself becomes memorable because Tom was the one to drive the car to town on that fateful night, thus Wilson had seen the car and when witnesses implicated a yellow car in Myrtle's hit and run, he remembers that it was Tom who had been driving it. Gatsby offers his bright yellow vehicle to Tom, who is already not pleased with the idea of going to town and is unsettled by being in Gatsby's presence. Gatsby is solicitous when he states:
Shall we all go in my car?” suggested Gatsby. He felt the hot, green leather of the seat. “I ought to have left it in the shade.
This the yellow car with the green interior becomes a character in the novel, an unwilling participant in the act of murder, and an accomplice to the events that lead to Gatsby's death.