I don't know much about computer illustration, so I'll go into the last two ones displayed that are hand drawn:
- First the contours are drawn with a water resistant marker or soft graphite pencil.
- Watercolors are added.
If you prefer to use non-permanent ink or want lines very sharp you should draw the contours lightly with graphite first, then color with watercolor and lastly trace the graphite lines with ink after the watercolor has dried. This will guarantee a very sharp black in the lines since they won't get obscured or smeared by the watercolor.
Actually, it looks like the second image in your post is using a non permanent marker line that has been added before the watercolor (do you see the smears?) – so if you want that look use a non-permanent one
The last image displayed in your post is drawn on a paper with a notable grain that affects both how the lines and water colors look.
I guess you would have a similar drawing process in a computer program as well: First lines and then add a layer of colors under that.
Hope this was to some help and good luck!