How do I do this in Photoshop?

[Hnilmik]

I can sorta guess at a way to do it, but it's probably stupid and a lot of guesswork and/or more complicated than I think.

My guess:
- Create a layer
- Go wild with the colors
- Make another layer with a bunch of "holes" in it
- Change the layer's blend mode

[GhostingFish]

I think for step 4 you might want to use multiple transparent layers and opaque coloring so that the colors blend and darken in the right areas to give it that cool psychedelic fashion that you are looking for.     Almost if you are using color filters.

[rtil]

if you're looking for a background that looks like the last panel, you don't have to do a whole lot. just get a watercolor-esque brush, splash some pretty colors on it, then on another layer make those shapes on white and set it to a different opacity. then overlay a texture above the 2 layers and scroll down the blend modes until you find one you like