This is the original sketch as submitted to the client. I added in a lot of background shading and some texture, because I knew the image was going to be heavily silhouetted, and I wanted to show a lot of detail and lighting to fully show the concept. Sometimes when you know a sketch is almost certainly going to get approved, it doesn't hurt to take it a little further towards completion then is ordinarily wise.
Here I have cut out the basic elements and started rendering the shadows and highlights. I have left the green goo completely flat, as I intend to render it separately from everything else.
I have started to add in the basic color, separating out his clothing and rags from his skin and bones. I've also begun adding basic definition and shading to the background.
Here I'm starting to add more of the scenes final coloring, putting in even more texture, and finalizing some of the little details.
And here I took another pass at the scenes overall color scheme, darkening it down even more, and adding a slight burgundy tint to the clothing to help separate the character from the cyan and green background. I also brought up the saturation on the green sludge in the background and at his feet, to really focus on the "pollution" concept.