A plastic grocery (liquor store/beer store) bag filled with every flavor of Four Loko which come in many assorted color cans therefore making the bag look like a rainbow. Four Loko is constantly releasing new flavors so it is hard to keep up with them all which is why a rainbow bag only needs to have at least all 5 of the "original" flavors : Fruit Punch, Orange, Uva (Grape), Watermelon, and Blue Raspberry. Watermelon and Blue Raspberry were not "original" flavors but are the 2 oldest flavors that weren't launched with the company.
John: What did you get man?
Tim: Got us a fucking rainbow bag!
John: Oh fuck, good looks son, were gettin' wasted tonight.