The primary process I work with is silk screen printing which can be confusing to try and explain without images. Here is the basics of a piece being made.
I blocked out the screen everywhere you see white, and when paint is applied, the color only goes where there isn't block-out. Layer one isn't anything to call home about
Things are getting a little more interesting with layer two. The same process as before was done; I blocked out everywhere I wanted the light blue to appear.
I'll admit, when I got to layer three, I thought that this was going to be a disgusting mess of mismatch colors.
I'll spare you the boring details, but the next six layers (creating a total of nine colors) went along the same way as the rest. Each layer was done through blocking out where I wanted to keep the previous color.
Only when the layers of space were completed did I clean out the screen. This removed all of the layers of block out. With photo emulsion, the negative space of neurons was added over the entire image, giving the loose rendition of space being confined within the neurons.