The original line art was done by Joe Mulvey 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.
Here’s a comparison of the original line art by Joe Mulvey and the flat colors that I did in Photoshop:
You can also watch the whole process here: