I'm having entirely too much fun doing this.
Make a new image and fill the background layer with a pattern. I'm playing with stripe patterns and this one I used a yellow and red one which I've attached below.
Any pattern will do though - or image or gradient - the sky's the limit.
Make a new, transparent layer and change the layer mode to Difference.
Fill the layer with color, pattern, gradient (try different gradient shapes while you're at it); try inverting the colors.
You will see a variety of different patterns as you change the top layer to different fills.
Also, invert the colors on the background layer for even more differences.
Here's one of countless possibilities that I made.

After merging the two layers, I put a pirate image on a new layer set to Difference mode and inverted the colors.
