I always thought that they weren't Aliens, that they were humans from the future using machines to change their appearance. They came back to study Dinosaurs.
In fact, when I first found this forum (Quite a while ago, but I didn't join), I read the unfinished story about some kids wishing upon a star, and ending up in the GV. I actually (at the time) planned to get an account and offer to finish the story. I was going to have the characters run into the Rainbowfaces, then at the end, do something with them (Like go into a strange cave or something) to get back to the present. Then they would all wake up where they were, with one character having something that would prove they were really there. Then, a park ranger would arrive, and tell the kids to leave, saying that this was trespassing. After they left, the ranger would get out a walkie-talkie, tell a "Mr Grosvenor" that the research was proceeding well, and he and a female ranger would head off into a strange cave...
(So yeah. Maybe the Rainbowfaces were spies for Universal. After all, they do have Doc Emmit L Brown on their side...)