Check out those fancy special effects. The other dimension isn’t very well defined, but since the other world is a big white void at this point no reason to not make the other one a big black void. The way I used to color the comic involved the polygonal lasso tool, slowly selecting the inside of each character in order to color them in with a flood fill. The color effect on the characters here works the same way, just instead of filling in the colors on a layer behind the line art I colored the line art itself. I’ve since stopped doing it that way, and I just use a Photoshop plugin that takes all of the lineart and created another layer under it with all of the different closed-in regions filled with a random flat color that I can then quickly fill in with the color I want.