What is a hook for an essay talking about green in The Great Gatsby?
You might consider addressing the various emotions and objects with which green is frequently identified, and then tying those to your discussion and analysis of the color green in the text.
For example, green is often associated with money. Money, its acquisition and whether it is "old" or "new," is of great importance in the society depicted by the text. Gatsby's money is "new" and thus considered to be of lesser value than Tom's "old" money.
Green is also associated with growth and innocence. Gatsby's belief that he can repeat the past is certainly an innocent and naive one as people have grown and changed far too much, especially Daisy, to permit such a return.
Green is also frequently linked to envy. To be "green with envy" is a common expression to describe someone as obviously jealous. Jealousy clearly plays a fairly significant role in this text: Gatsby is jealous of Tom, Tom becomes somewhat sort of jealous of Gatsby and Daisy's feelings for him. These feelings prompt much of the action leading up to and during their confrontation in New York City.
Your hook might discuss any or all of this symbolism as a way of engaging the reader's attention.