Relieved that I can buy it with free currency

I pulled until the very end of the Brave Pass, so I have no currency and can't exchange it now, but I'm glad it's not limited to paid currency. Maybe if I buy a song and get all rainbows on all difficulties, some will come back...? I've never gotten a rainbow before, so I don't know how much I'll get, but if it's like gold, it's probably 60 for each difficulty, so 180, meaning about 30 will come back, so maybe exchanging songs other than my fave is actually a good idea...??