I think racism can occur in a Hispanic/black scenario, but there was nothing to imply that any racism had occurred in the Zimmerman/Martin case at all. You're right though, Zimmerman was portrayed as a white man, and I think a bunch of people just automatically assumed he was white with a name like George Zimmerman. Here, it's a black kid who killed a white guy, and he even said on his Twitter that he doesn't like white people.
Even if he's only joking, he probably has a lot of resentment for white people to make a statement like that.