You know you get free RCs (20 for silvers and 10 for gold+ units) by simply rolling gacha. So for every silver you roll, that's 20 RCs added to your RC count. I rolled two golds today so I got 20 RCs. Not sure if this is a permanent function.

Chose B.Iris for my black ticket. Max skilled her after 4 rainbows and -2CRed her after two black CR fairies. Also got Lucia, Ashela, and Fiore to 5/5 skill after one rainbow. Guess I'm lucky for getting 4 units to max skill level with a minimal amount of rainbow fairies used.