When a person eats really shitty Mexican food so that a few hours later he or she - but mostly he, since girls don't poop - simultaneously shits and vomits from both ends. Once the purging begins, the person will get on their hands and knees, and the spewing will look like a rainbow.
Man, last night I went to Vallartas Mexican Restaurant and I had the worst taco and refried beans EVER! When I got home I redecorated my bathroom with a Mexican Rainbow. It was so gross!