Its a hack of Champ Edition, really. One of the many bootleg hacks before Capcom came out with Hyper Fighting.
There are a couple of versions of Rainbow as well. The most prominent way to tell (other than the obvious splash screen) is if you hit start during a fight, your character will change to another character. It's always the same lineup of changes, but still, its pretty cool. It also has a couple of different fireball speeds. Incredibly slow, regular, and super crazy fast. Also, depending on the rainbow version, Ryu/Ken's fierce shoryuken will travel a huge distance, horizontally. It's probably my favorite of the Champ edition hacks.
I don't believe they're too rare (could be wrong tho). I had one a couple years ago that I don't even think was in MAME, and ended up selling it for I *think* $70 or so.