The color green could be symbolic of goals, particularly Gatsby's goals. in the beginning of the book, Nick mentioned something about Gatsby staring off into the distance, across the bay where there was a green light at the end of Daisy's dock. That simply could be symbolic of his goal to once again win her love.
The color grey could symbolize the downtrodden, poor, lower class, or maybe even foreshadow the terrible things to come. For instance, in chapter 2 (I think, or wherever the reader is introduced to the Valley of Ashes) Fitzgerald uses imagery to paint a "grey" picture of the Valley of Ashes. This correlates with what the Valley of Ashes stands for.
In terms of the other colors such as white, red, yellow, and pink. I'm not quite sure off the top of my head. Typically, red means passion, anger, or love. Whereas white typically means purity. However, pink, yellow, and white could also have something to do with Spring as a season or perhaps the idea that love prospers in the Spring, which is why flowers bloom, etc.