For the "plague" assignment, I decided to focus on the bacteria (yersinia pestis) responsible for the Bubonic Plague and other human illnesses.
Original graphite drawing
Bringing it into Photoshop.. I added a background color, and changed the line color by selecting all the white in that layer through the channels palette, and deleting it to isolate the lines.
Here I've begun playing with color, keeping each fill and line color on its own layer so that they are easy to change. Since I wanted two different line colors, the greenish line color you see here is a layer above the blue lines, and turned into a clipping mask. I also added a paper texture from cgtextures.com (just a layer set to multiply over everything).
After a lot of experimenting I ended up with a much bolder color palette.
Finally, I added another layer of texture, a bar of color to call attention to the typography, and a border to anchor the illustration in place on the page. Not totally sure if I prefer this version over the previous or not...
Here are my layers at the end!