When I saw that you could get a different-colored Sim from a malfunctioning tanning booth, I thought this might be a way for two beige or brown Sims to produce a green or orange, etc. child.
I took a family with a teenager and gave them a tanning booth. The teen pranked the booth so the mother came out lavender and the father orange. The parents then tried for baby -- which turned out normal.
So, in case anyone has been wondering -- pranking the tanning booth will not result in a rainboo.