Pink can represent caring and compassion. It is a combination of red (here, we might use red as passion) and white (representing purity and hope). The pink color then represents passion, purity, and hope. Recall that Daisy and Gatsby (Gatz then) had a relationship before money became a real issue; it was therefore more pure. Now, their reunion recalls that purity but it is now more a balance of passion and hope, a hope that can not be totally fulfilled as long as Daisy is with Tom. 

The fact that it is a pink cloud is significant as well. Prior to the quote in question, the sky is described in narration: 

The rain was still falling, but the darkness had parted in the west, and there was a pink and golden billow of foamy clouds above the sea. 

The darkness parts: the sunlight is coming through (from behind) the clouds. It creates gold and pink hues. The gold is associated with Daisy, Gatsby's American Dream, and money. The pink symbolizes his passion and hope. The pink and golden clouds also seem like a dream; this alludes to Gatsby's dream of being together (hope) with Daisy; but there is also the idea of a dream being imagined, not real. Daisy (and Gatsby) would like to live in that moment, make it stretch in time. However, the sun will eventually go down and the pink and gold will dissipate. 

Generally, light colors in "The Great Gatsby" represent dreams or goals. Gatsby himself wears either white or pink suits, which seem to represent his pure, and naive nature. However, the color imagery seems to go a little further in the scene where Daisy mentions she wants to "push him around" in the pink clouds. Just prior to this comment, Gatsby has mentioned the fact that there is a green light on her dock that shines all night. Although both Gatsby and Nick realize that Gatsby must have stayed up all night looking at the light, Daisy seems oblivious to this admission. She simply goes to the window and sees pink clouds. Fitzgerald is indicating though the image of pink clouds, that Daisy is the one who seems naive this time. In addition, she is also insubstantial as a cloud because she has totally ignored Gatsby's obvious obsession for her and simply wants to "push him around".