I'd never heard of it either. They used grulla instead of grullo to as someone pointed out. I don't buy just for color, so it was a plus and definitely a gimmick I think. I looked at the little guy and I do love him, he's very respectable and has good manners for a youngin' but I do NOT like the ranch where he's at. They ponied him with a child on his back and are working him way to hard for his age. He is registered and definitely has the grullo color maybe not "black".
ANYWAYS, do not want to hijack the thread. I am a sucker for the grullo/grulla color. Just love the looks, especially if they're on a good horse. (: