Rainbows panels are very good for Earthworm and useful on map with enough space for the 1.3 skill act bonus.
I think they made it just because they made Chocolate Cosmos 6*.
For event FM, I tjink it's better to keep 5 lvl 10 instead of one lvl 50. For stats bonus there is the other FM. +20 speed over 5 girls is better than +50 on one.
I don't remember the effect of Bluet FM. Someone know it?