Assuming that you're using Paint Tool SAI for the colouring, you can add a greater sense of depth and detail by using one of the 'watercolor' textures, that comes bundled, on your colour layer.

Well, at least it came bundled with my copy.



On something like Photoshop, however, you'd probably want to get some form of paper texture and apply a bit of noise to it; then, impose it onto your colour layer by any means of preference.

That way, not only have you added an extra touch of depth but you are also keeping it more consistent with the traditional background and linework you've got left over. It gives more of the impression you actually coloured the artwork through traditional means.