Purple is not a Grumble. He is something else. He isn’t an animal. But he’s not a Grumble. He doesn’t talk, and he doesn’t have stick limbs, only two, wider, elephant-like legs. Also, his name doesn’t fit with the RGB/CMY naming convention. He is an outsider, and no one knows where he came from.

You can also see Green’s eyebrows here. Most characters besides Red and Yellow don’t have their eyebrows drawn, but they all have them. I don’t know if they have hair on their eyebrows, but since they are capable of growing beards it would make sense that there is some hair on there somewhere.