The original line art was done by Carlos Gomez of DeviantArt. All I did was vector the lines to give it a solid look and initially apply flat colors. Only after the flat colors were done, I changed the colors to B&W to emulate the look and feel of an actual inked line art.
Added a few touches of my own as well in the background. Notice the picture frames/posters?
Here’s a quick comparison of the original line art by Carlos Gomez and the flat colors that I did in Illustrator: