watermelon - green on the outside, red on the inside - as Foolestroupe said, its a kind of reds under the beds comment, as in they're pretending to be Greens (Green party members, environmentalists) but watch out, theyre all closet commies.

another similar one is used in the Koori (aboriginal) communities. If someone is aboriginal, but has no loyalty to the community or is a wannabe white boy, thyre called coconut (black on the outside, white on the inside).