Rainbow cichlids or the non-cichlid rainbows? If the former, I don't think they care at all. Mine are quite happy on light colored PFS, although they like to put their eggs on dark rocks, for reasons that escape me, but seem important to them.
Mine lay their eggs on rocks, flower pots, and leaves of amazon swords. They will move the fry after the eggs hatch, into the sand or even hang them in the roots of floating plants.