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!
Rainwater mixed with oil from several junky mexican owned vehicles leaving a parking lot, creating a rainbow-like effect on the pavement.
José and his homies left a huge Mexican Rainbow in Wal-Mart's parking lot.
A variation on the Dirty Sanchez sexual move, where the first participant eats a box of Lucky Charms 2-3 hours beforehand. This results in a more colorful and amusing feces.
Josh (on the phone): Hey girl, I was thinking we could try out the Mexican Rainbow
Josh: Awesome, let me just go to Walmart and pick up some Lucky Charms. Ben's going to be so jealous when he finds out!