We don't have synonyms for rainbow kiss yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are rainbow fish and rainbow kick.
(n) :
(slang, sex) A supposed sex act in which a man and a woman who is menstruating simultaneously perform oral sex on each other, followed by a kiss where the couple mix semen and period blood in their mouths (which makes the so-called "rainbow").
(n) :
(slang, sex) A supposed sex act in which a man and a woman who is menstruating simultaneously perform oral sex on each other, followed by a kiss where the couple mix semen and period blood in their mouths (which makes the so-called "rainbow").