According to the shop... each rainbow only required 6 coins to buy... not sure what you are talking about here.
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Nutaku neglected to add the main source of rainbow medals: leader tickets. They should be bought with leader medals and have a 92.5% chance of giving a 5*/7.5% chance of 6*. If you get a 5*, you also get 30 rainbow medals.
I wouldn't hold my breath waiting for aqua shadow fragments to be added.
For those wondering, the 100 stamina mission for red/blue manyus is working fine.
I think you get 50 for a gacha dup 6*. I got 52 total right now.
looking to get Anemone someday
When I get another 17 from the Nations, I will try the 11 pull again.