According to search results, a "rainbow kiss" is a type of kiss that can only be performed during intimate relations with a woman whos experiencing her monthly menstrual period. The "rainbow kiss" involves the mixing of both menstrual blood and semen, putting them on the persons lips and then kissing. The term has gone viral on TikTok, with many users reacting to the explicit meaning of the term. It is important to note that this is a sexual act and may not be suitable for all audiences.