/vr/ - Retro Games

What is the best way to remove the cyan color from an image? I think this is an issue because I use PNGs as my sprites.

I've found a bunch of old sprite sheets that use this background and it'd be great if there was a quick way to remove this color.

I have photoshop and paint.net but I can't find a clean, quick way to do this.

what am I doing wrong?