If the yellow color is the color to show Gatsby’s outer self, the blue color, which is full of sadness and fantasy, indicates Gatsby’s real inner self - lonely, sorrowful and fanciful. In the novel, Gatsby’s garden is described as blue. The blue tone of Gatsby’s garden reveals Gatsby’s loneliness and sadness in his inner part. He hosts insane parties on his property to impress Daisy but he just fails, which makes him even more sad and lonely. So he hosts those big parties but doesn`t attend at them. I think a reason for it might be that the real Gatsby cannot identify himself with those wealthy people and the tranquility and sadness in his inner heart is the complete opposite to the extreme