Discard pile, color change?

I’m looking at editing a module that needs a bigger distinction between the draw pile and the discard pile. This game the discarded cards are placed face down so what was discarded isn’t known by the opponent. So the draw and discard piles look identical.

Ideally I’d like the cards that get placed in the discard pile to turn black and white. Is there a way to do this?

A more easier idea was to make the border around the boundary of the discard pile to be different than the draw pile- like a thick red border. However once you discard the first card that border is covered up.

So B&W idea or a different solution perhaps?