I am having trouble understanding why the rainbow pearl is a rare card. It seems to me that the only time it is useful is when you are way ahead in the match and you really don't have anything else that you would rather play. That way you could save the power for later.
Other than that it seems like it's actually worse than the shimmering pearl because it costs 2 and, in effect, it still only gives you 1 extra power since you have to use 2 power to even play it, whereas the shimmering pearl is a 0 cost card. I can see it helping the ai if there is a power card that gives them a free item or something, but I fail to see a good use for it, from a player perspective.
Maybe I'm missing something (and if I am, let me know), but it just seems like it would only be worth using if it had one less cost or it gave one more power. In any case, the game is fun and I play against my brother all the time, so keep up the good work. I look forward to trying out the air expansion.