I accidentally got some clear nail polish on a library receipt and the whole mark turned dark. Then I put some red polish on, and the edges turned dark. It doesn't work like this on normal paper; why?

If I am thinking right...the receipt paper was from a credit or debit card machine and two copies come out of it at the same time...(for signatures/ or to be signed) one copy for you and one for the vendor...Probably carbon based.