your nailin a girl while she is on her period (with a condom of course) and you shoot into the condom, then you make her drink the contents out of the condom like it is an oddly shaped glass. 'rainbow' comes from the idea of many different colors all draining into her mouth.
my girlfriend was on her period, and she really wanted to fuck, but i wasnt really feelin the whole blood-thing, so i told her i would only if she would 'taste the rainbow.' that bitch slurped it up like the final parts of a milkshake haha.
by vic tha sick dick June 28, 2006