The only problem, well not exactly a problem, is that the girl who you want to rainbownize is usually your waifu but normally you should already use dressbloom/skillbloom on them. Some might even use ampy on them but I guess it shouldn't be a problem since amped stat should be carried over. The point is ideally you want to rainbownize from fresh girl right. As I said, very clever strategic marketing of dev team.