I usually colour my sketches (or even the cool sculptures on here) with Photoshop, I find this really easy to do and you don't really have to worry about messing up your previous work and at the same time I will usually edit the actual line art for the sketch as well. I usually scan my line art into photoshop, adjust the brightness and then maybe trace over the work a little with a translucent black brush on a separate layer, incase some of the details are missing. Then I'll add another layer for my colouring, which I'll set to the colour layer style; This allows the colour in the layer to be applied to all layers beneath it, but it won't have any affect on the brightness/darkness of the layers. Then I'll probably make one or two layers beneath that colour layer to fill in the line art so that the blank spaces aren't too bright or dark.