In chapter 6, why does Daisy say she's giving out "gren" cards?
I believe that the "green cards" are a reference to dance cards, which were a social custom at the time. When a woman attended a party where dancing would occur, she could hand out cards to men with whom she would like to dance. Another similar custom was a dance card or booklet that she kept, and potential dance partners asked to sign it so that they could dance with her. This custom is very different from modern dance behavior. The color green reinforces Fitzgerald's use of that color to represent hope and wealth as in the green light at the end of Daisy's dock. Daisy comes to Gatsby's party eager to have a good time, of course, but she also looks forward to seeing Jay in a very public place where their private behavior may go unnoticed. She's almost giddy with excitement because she offers Nick a card, telling him he can exchange it for a kiss, which would be, from Nick's perspective, pointless.