I play Spectrail almost everyday whenever I do Major Drills' quests and I've gotten prisms plenty of times, but I have never once gotten a rainbow.

Is it even possible to get? If so, how many points is it worth?

I honestly haven't seen it myself, but a quick google search brought me to an old thread that says people have gotten it and it's worth 500 points.

Unfortunately the thread they linked to for a guide for the game, no longer exists.

Thanks! I'm still skeptical as to whether it's possible to get or not, but maybe I'll be able to find one if I keep playing Spectrail for Major Drills eventually. XP