Colors are an important part of the symbolism used in telling the story in The Great Gatsby. Color references appear throughout the book.
White is the color of purity, of innocence. The first time readers are introduced to Daisy and Jordan, they are described in almost angelic terms.
They were both in white, and their dresses were rippling and fluttering as if they had just been blown back in after a short flight around the house.
Gray is the color of hopelessness, of the houses and businesses and people trapped in the valley of ashes. Grey is also used to indicate things or people who aren't important.
Green, most notably mentioned in connection with the "single green light...that might have been the end of a dock," is the color of life, of hope, of looking forward to a better day. Ironically, Michaelis mistakenly givens an intial report that the "death car" was "light green."
Blue is associated with depression or unhappiness. Gatsby's lawn and gardens are blue, to match the color of his mood when despairing of every fulfilling his dream relationship with Daisy.