In "The Great Gatsby," I heard that three colors (Green, Purple, & Gold) are symbols for something. What do these colors mean?

Colors have meaning and are capable of being symbols just as easily as any object or person.  With regard to The Great Gatsby, Green represents Gatsby's dreams and his wealth hopelessly tangled together.  He gains one in order to gain the other, and in the end he loses both.  There is a green light at the end of Daisy's dock, which suggests Gatsby has kept his dream of the Daisy he knew and loved before he made his fortune, and the fact that their relationship will always be affected by money.  Nick associates Gatsby's dream and the green light to the American dream, "a green breast of new land" (189) where people could come to begin anew.

Purple is the color of indulgence. The rooms of Gatsby's mansion are decorated in lavender, and he wears lavender shirts.
One of the twins in yellow also mentions that he sent her a new dress with lavender beads after hers was ruined.
Lavender represents the needless things that the rich characteristically do.

Gold is one of the main symbolic colors in The Great Gatsby. It represents the wealth and extravagance of the rich. It also demonstrates how careless the wealthy can be though they are considered "high class" with all their money. Gatsby's car, with which Daisy kills Myrtle, is yellow. Yellow is also a shade used to connect a person's snobbery with her money--Daisy is referred to as "the golden girl," Jordan's hair has an "Autumn leaf yellow" tint, Gatsby wears a gold tie.