Old Pepsi and other soda Color Keys

You might not be able to help me but I thought I would at least try. I have found a few boxes full of color keys. My understanding is when a can of soda is about to be made, they make color keys for the artwork as proofs. My color keys have different layers of plastic, one color per page, and when they are all laid on top of each other they make the whole can artwork. I have some that have been discontinued and are really rare. Does anyone have any idea of what I am talking about? I was wondering how much they might be worth, I don't plan on getting rid of them but I can't find a thing on them.
I thank you for any information you might be able to give to me.