I've been experimenting with photo polymer etching in my digital printmaking class. For these two I made two separate channels for each of the colors, and then erased parts so that I'd have part that were pure blue, pure yellow, and then a mix of the two where they overlapped. Then I printed each channel out on a transparency and exposed them on top of a plastic plate with film adhered to it. After developing the film, there are little grooves in the plate wherever the black on the transparency was, and from there on you can ink it up like a normal etching. These are two of the more "perfect" ones.