There aren't that many redheads in the MU, though, so the artists can get away with a lot. Basically, if you see a beautiful redhead, it's either MJ or Jean Gray, and you can usually tell which is which by what comic you're reading at the time. If it's a Spider-book, it's probably MJ; if it's an X-book, then it's probably Jean Gray. If it's a Spider-Man/X-Men team-up - welllll, it's STILL probably Jean Gray, because the writers generally keep MJ discreetly off the page during such things. Saves embarrassment, donchaknow.