from the FAQ
"V - What rainbows was Cartman talking about?
Cartman had a wonderful rant about rainbows in the first scene of Weight Gain 4000. This isn't a rant about anything in particular. Matt and Trey heard a little girl talking about how she hated rainbows one day & thought it was so odd that they put it in the show. Personally, I think the rainbows are a allusion to gay guys. The rainbow is a common gay symbol. 'Tossing salad', for those who don't know, involves the sucking of the ass <with syrup or jelly>. I find it very possible that a guy could bite another guys ass in the throes of passion, or that Cartman may just describe it that way. Some of you may be thinking, 'Why would someone be sucking Cartman's ass?' Well, Cartman's mom _is_ a slut, so there's all kinds of guys running around at any given time in their house."
I'm not even going to ask how the author of this part of the FAQ knew all this stuff....
I thought Cartman might be talking about the Rainbow Warriors, that bunch of people who goes around saving the earth and getting their ships sabotaged by members of French Intelligence. Reason? Cartman hates hippies and the Rainbows are pretty famous hippies.
Anyway, it's just a guess. Cartman's rants don't always make a lot of sense either in the context of the particular South Park episode or generally accepted reality. It fits in with my theory that the vocal overdubs are done late at night, in one take, by some extremely stoned individuals who are reading from a handwritten script, and not always the script for the show they happen to be doing.