As you can see, the background hasn't gotten any shading yet, it's just pure lines. There is a reason for that, which is that I want to put down flats (on a new layer) before shading. That way I can create a selection from the layer, and not get the background mushed up with the character.
I start by giving the entire background the same color. That way I can lock the layer transparency and not have to worry about painting on the character. The background layer has to be underneath all the character layers, that way it won't mess up his hair (I'll get back to making it look better by the way). It also needs to be above the soft light (tinting) layer, otherwise everything will turn odd and blue. I set the layer's blending mode to multiply, just as I did the character colors. In regards to the hair, you need to get some paint underneath it, for it to look like actual strands of hair. This will look horrible at first, but will be fixed later.
Fixing the hair edge
Much the same way as the edge between skin and hair was fixed back with the character, I carefully smudge the background a little, to make it look more like it is actually underneath strands of hair.
I start off by laying down some basic shine, using an add glow blending mode, much the same way as I did with the metal parts on the character.
Bringing it all together
I think the initial gray of the background clashes with the shine, so I create a new layer on top of both the flats and the shine, and set the blending mode to color. Making a selection from the flats of the background, to make sure it doesn't affect the character, I flood fill with a blueish color.
Seeing as the bolts were initially golden, I zoom in and erase the blue tinting from those with a hard eraser.
The chair is super simple, I just create a new layer, set it to multiply, and color in the same way as I did the clothes.