Not sure of what you mean by the “under drawings”, but I think you refer to what I’ve sometimes called “the shadow layer”. Well, I don’t always use the same technique; it depends on what kind of effect I’m after and also on my current degree of laziness. Mainly the latter, to be honest. Some common options are:
- The brush tool in Photoshop
- Gray markers applied over the pencils, before inking
- Indian ink (or whatever other medium) applied on a separated sheet of paper, then merged in Photoshop.
- Screentone (I mean, physical sheets of screentone) applied over the inked piece. I use the ones sold by Letraset.
Not on purpose, but I think I listed them according to reverse order of awkwardness. If you browse some of my former posts you will find examples of all of these techniques, some of them with a detailed description of the process (well, sort of).