Good point! I forgot to ask about that.
If the rainbow you're talking about is a wavy vertical pattern that has a slightly multicolored hue from certain angles, part nga yan ng tempered glass. You won't be able to do anything about it.
However, if the rainbow you're talking about is the tracks left behind by your wipers -- then you may be able to do something about it. Kung hindi naman siya gasgas na galing sa wiper, then malamang water mark + silicone + road grime buildup lang yan. The Elevo will make short work of that...

However, kung gasgas yan from the wiper, you'll have no other recourse but to have your windshields buffed/polished.