I've no idea what mod that is, sorry, and there are two "rainbow" spells in the game: Color Spray and Prismatic Spray. Color Spray is useless because it only affects creatures so weak, you could have killed them by sneezing anyway. Prismatic Spray is mostly useless because there's a chance of it turning people to stone and thereby destroying almost all of their loot.
Making spells can be easy to moderately difficult, depending on what you're trying to do. After you download DLTCEP, start by opening up existing, working spells, saving them under different filenames, and start making small changes. As you go on, you will screw up, learn the "how" and the "why" of your screwups, and eventually you won't suck all that much. Just like me.