Board Thread:General Discussion/@comment-30487496-20180514113436/@comment-31661832-20180528235629

There are plenty of cards to invest orbs in. In fact, you'll probably never have enough orbs to rainbow everything you have.

If you "don't have anything to spend them on" then you're just too picky about what units to spend them on. Rainbow a F2P or a card you have a special attachment to. I was particularly thankful for those orbs, I rainbowed AGL Super Vegito and the AGL SSB Goku with them.