Who is a cartoon character with a rainbow? Cheer Bear of the Care Bears! She has a rainbow symbol on her belly. :) You can see a more simple Care Bears nail art design here that I did last May.I struggled with what to paint on the other nails because I didn't want to do a rainbow all the way across them. I settled on a blue sky with some puffy white cloud 1/2 moons.
I wasn't physically able to get my fingers in position to line up her head and tummy. Whoops! That's ok, we'll just make a collage and pretend!
Tutorial? I compiled them into a GIF, and I'll spare you all the same details of pretty much every cartoon I've done. A polish base, and then acrylic paints, etc.