Yellow
Gatsby’s car is yellow. Yellow is like a fake version of gold. Therefore, his car is yellow as a representation of his illegitimate fortune as well as his fake life. Everyone here has heard the gossip about Gatsby, so it’s safe to assume that he probably did something wrong to get that money.
Anytime you see gold, it is safe to assume that the person is real and rich. However, yellow is a representation of trying to be something you’re not. Here in the 1920s, almost everything is yellow because almost everyone is shallow.