Colors are prominent symbols throughout The Great Gatsby.
Green represents hope, vibrance, excitement. The green cards Daisy was giving out at the party represented opportunities to meet someone new, to share in a brief fling with someone new and different and exotic. Daisy was hoping for some exciting and romantic new adventure during the course of the party.
Gold is the color of wealth. Daisy acquired plenty of it when she married Tom Buchanan. In giving the gold pencil to Tom as he was going to join another group of people at the party, she was helping him to be ready in case he needed to make note of any important information that might be beneficial to them at some time in the future - "if you want to take down any addresses here's my little gold pencil."
Gray is often associated with things that are unimportant or hopeless. The "gray haze of Daisy's fur collar" is mentioned as nick and Tom are arguing about Gatsby's background and the type of people who had been at the party; Daisy joins the conversation to disagree with Tom and defend the others attending the gathering.
Who is this Gatsby anyhow?" demanded Tom suddenly. "Some big bootlegger?" "Where'd you hear that?" I inquired. "I didn't hear it. I imagined it..." "Not Gatsby," I said shortly. "Well, he certainly must have strained himself to get his menagerie together." A breeze stirred the gray haze of Daisy's fur collar. "At least they're more interesting than the people we know," she said with an effort.
The "gorgeous, scarcely human orchid of a woman" is a movie star; her beauty is being compared to that of an orchid, that most beautiful of exotic flowers. The white of the white-plum tree she sits beneath represents purity and honor. Daisy and Tom are in awe of her and of their great fortune to be introduced to her.