Originally Posted by Zeban

Rainbow coins are kind of a gimmick, imo. If you draw a duplicate 5* you get 2 coins. If you want to do enough gachas to get 150 dupes, you're probably able to get the 6* you want through some other means first. It's still a cool little bonus, but definitely not something I'll ever try to grind.

Rainbow coins are pretty useless
I just added all my dupes and if I got rainbow coins from when I first started playing which was over a year ago I'd have about 22 rainbow coins
If it was 100 coins I could understand that even if 50 dupes is still a lot

I'm guessing there is going to be daily deals/special gacha which give rainbow coins but 300 is way too high to care for