A party where girls each wearing different colored lipstick
give blowjobs to the guys, the marks left on the guy's penis are the colors of a rainbow. Contrary to popular belief, this was not a term made up by Oprah, it was brought about in a juvenille fiction book called "Rainbow Party."
*washing hands in a restroom*
guy 1: dude what's all over your hands?
guy 2: oh I just left a rainbow party
guy 1: man you rock