I posted the following up near the top, back on 5/2, mentioning the issue of throwing wheat. I obviously made a mistake by saying a symbol of fertility, as siman bracha makes much more sense. I think the whole thing got combined with goyish customs as far as combining the wheat kernels with confetti and then finally leaving out the wheat altogether (though I have still done that at a few chassanahs in the last 10 years) possibly due to issues with chometz in the chosson’s streimel, as I mentioned in my original post. Thus, I do not believe that there is a mekor for throwing confetti, but it did start out as something Jewish that morphed into doing something stam/goyish.
“I have been at many chassanahs where they do throw “confetti” on the chosson at the badeken. I was told that the original version was to throw wheat kernels as a symbol of fertility. Later that got mixed with confetti and possibly due to issues with chometz (famous story with R’ Shmuel Salant and the wheat in the soup on Pesach) many people do not use the wheat anymore and just throw the confetti. I see this less and less these days. it does make a mess on everybody.”