He's grumpy. He's mean. He beats up the gangstas. He's the mouseman. Pen on paper. Colours and textures added in photoshop.

I've finally found out how Ashley Wood (most probably) colours his lines. In photoshop, you add a layer above your b/w line drawing, then set it to "linear dodge" filter. Then colour in that layer. The line colour will change depending on which area you add colour to.