I'd probably find fabric with rainbows and work with the fabrics she picked out for alternate blocks.
Just because I can never remember the mnemonic, here are the colors in order:
red, orange, yellow, green, blue, indigo and violet
A number of years ago I designed a "rainbow trout" block, bars of the rainbow for the body, plus a couple of triangles for the face and a couple for the tail. That doesn't sound right for your niece but it was well loved by the son of a fisherman I gave it to
