Originally Posted by Taon

You typically earn rainbow medals whenever you roll a 5* (2 RMs) or 6* (50 RMs) on a gacha. Occasionally various deals will also give RMs, such as the current special gacha (150 RMs per 2000NG spin).

Originally Posted by fghk0

The key to getting a rainbow coin is to repeat

Now that you mention it, yeah I just remembered that I got it with my repeated Anemone (Miko). And yes I know about the current gacha but its too expensive for me right now, I'll wait for a better offer.

Thank you for your answers guys you are really helpful.