Originally Posted by amoebapro

I see, well I only have 2 rainbow medals aaaand I don't even remember how or where I got them.

Also It's good to hear that they are not going to expire or something I guess, but at this rate how long its gonna take to collect 300?

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).