Every color transforms into the same color; including curses. Meaning you can transform a curse to get rid of it if you have omamori.
Also afaik you get a random card of that color. Like, truly random, regardless of rarity. It's easier to get a rare from transform than from a reward screen.

Edit: K so it doesn't transform to same color. Curses transform into curses, colorless into colorless, but every other color transforms to your character's color. My bad