Rainbow kisses are a specific type of sex act that involves both semen and menstrual blood, and those who actually do it typically do so because they share some sort of fetish for bodily fluids. According to an interview with Dr. Wendasha Jenkins Hall, Ph.D. in Cosmopolitan, a rainbow kiss likely evolved from period sex, which involves penetration or oral sex while the biologically female partner is on her period.

Performing a rainbow kiss is really just a combination of a couple of different sex acts. It involves both partners simultaneously performing oral sex on each other while the person with a uterus is on their period. Once both partners have ejaculated into each other's mouths, the members of the couple kiss, mixing the menstrual blood with the semen and creating what users describe as a rainbow.