Cards Colored with Yellow Marker?

[BrunoThePug]

I was playing in one of the tournaments at Fort McDowell casino in Fountain Hills AZ and noticed that some of the cards, maybe all of them, looked like the middle part of the card had been filled in with a yellow Hi-Lighter. They were using jumbo index cards (like blackjack) so I wonder if that had something to do with it.

Is this a common practice? What's the point?