Keeping with the theme of all Prepare cards being conditional on something (as Vitenka thought over here), this is a Manamorphose you can set up one turn to use later. I like this idea because it allows you to seem tapped out while having two prepare cards face down, one of which is this and the other has a prepare cost that you can pay with this. Or you could just use the mana from this to cast a regular combat trick, I guess, as long as you have something else prepared.

I'm considering making this green. The flavour fits either way, though the Black Knights in red are the best at making secret preparations. Ritual-style effects are more red than green, though the rainbow tugs this slightly more greenwards. Red has a lot more to do, though, while I'm somewhat searching for green things to do; this would also help the (Green-white archetype), especially if I drop this down to common now the hybrid free cycle are up at uncommon.