Nitrile C-Rings Rainbow from Spartacus Leathers. Colorful, flexible Nitrile cock rings from Spartacus are the perfect skin safe solution for those with rubber allergies, and they offer a firmer experience than our rubber rings. Nitrile Rainbow Ring Sets in response to the popularity of our rainbow pack of rubber rings, we have introduced nitrile variety. Colorful, flexible nitrile rings are the perfect skin-safe solution for those with rubber allergies, and they offer a firmer experience than our rubber rings. Each set includes 5 rings of a specific size, but different colors. red, blue, purple, white and black. Phthalate free and latex free. Strong and flexible. Compatible with all lubricants.