I've never played Guitar Hero and this image wasn't for Activision. It was for a legal trade magazine for a story about patent law; so something dull and something exciting (if you're into that sort of thing, I'm perfectly content with my New York Times Crossword Puzzle app and maybe a little Brave Man now and then).
I wanted the image to be about both situations, the real and the imagined, so I combined patent writing with guitar playing. The patent attorney is writing while he imagines himself playing guitar.
For process, first I scan my sketch and convert it to bitmap to get rid of the gray pixels. Then I move it to a new layer and delete all of the white pixels. I lock the transparent pixels so that I can quickly change the color of the sketch using the Fill Command shortcut (Option-Delete to fill with the foreground color, Command-Delete to fill with the background color). Below is the sketch with some reference photos taken by my daughter Emma.
Next I introduce a background color and then additional colors, working at separating the foreground elements from the background. I use the Pencil and Eraser tools on Layer Masks to isolate the color areas, working quickly trying not to be to accurate or neat. I make sure all anti-aliasing is turned off (uncheck anti-alias for the Lasso and Magic Wand tools, set the Eraser to Pencil, avoid the Paintbrush, set Interpolation to Nearest Neighbor). This limits the color areas to hard edges which allows me to do quick color edits when I lock the transparent pixels on each layer. I work fairly large, 11x13" at 300ppi so no jagged edges will be visible on curved and angled edges.
After these initial steps, I spend a bulk of the time cutting and pasting in textures and adjusting the color palette as needed. I often add additional line work as well.
When I have the image design complete, as a final step I convert it from RGB Color to Indexed Color, limiting the palette to just a few colors, and then tweak that version a bit to fall somewhere between the RGB version and the Indexed Color version. Below is the final image design followed by the Indexed Color version. The first image in this post is the tweaked version of the Indexed Color image. It's the final image, the one sent to the client.