I have been very interested in finding ways to combine ink lines, grayscale paint washes and digital color. It’s very simple and effective to digitally color linework, and very simple to apply painted tones to a traditionally inked drawing. The challenges come when you want to do clean color fills to a traditionally inked, painted and scanned illustration. It can take a long time to color a piece like this because selecting the edges quickly and cleanly for color fills is not as effective when there is a lot of value subtlety
My solution to date, has been to work from a pencil drawing on a paper that can handle ink washes. Once the pencil drawing is established, I would paint using acrylic ink to add value washes, texture or other effects. I was using a very textured watercolor paper so that the washes would have a lot of character. Then, I take a piece of vellum and tape it in place over the painting/drawing. Vellum is an excellent surface for traditional inking with a brush and it’s extremely transparent, which makes it easy to ink without a lightbox (although that is a good option too).
What you get in the end is one ink drawing and one toned drawing, both traditionally produced from the same source drawing. This is a bit of trouble, but it allows you to scan the two parts separately, adjust each scan for its particular needs and then composed them together digitally.
Why do this? Because I like the adjustable capabilities of a flat digital color-filled line drawing AND the texture and subtlety of a traditionally painted values. I also want to see if this is a practical way to work.
Why not just add a random, stock-photo watercolor overlay on your digital color? I’m not in the habit of asking questions like that.