@Eric-Castleman Sounds like you are painting like Peter Stapleton does here: https://youtu.be/6ocGms3cElg (He adds color to the b&w painting at 01:01:10 into the video)
You can definitely do an underpainting and then paint in low opacity or “roughly” on top so that the underpainting shows/peeks through the lesser opacity areas or the “holes” created by painterly brush strokes. I have found this difficult to do digitally due to the way I have tended to paint; however, it is something I am starting to experiment with to find "my way" to do it.
another approach you can do—if you are wanting a tint of color to permeate the entirety of the piece—is to paint everything according to whatever values/colors they have then above all of that, create a layer that is flood filled with whatever the tint color is to be, change the layer type from "Normal" to "Color" and reduce the opacity (likely reduced way down). You can even do multiple different colored layers at differing opacities if you need to give color nuances, and/or mask out areas of the color layer (whether a harsh shape or a gradation from one color adjustment layer to the other(s))
This will give an overall tone to the piece, but it isn’t quite the same as what Will was showing, since the tone would be even rather than peeking out here and there. Though, I suppose you could do apply some sort of texture mask or paint out/in according to where you want the tint to go or be hidden…
Basically, this other option I’m sharing is the inverse process of what Will was doing, by adding the tint on top rather than it being on bottom like in real-life painting. One advantage is that you can turn the tint layer(s) off or change them to another kind of tint…
Just a thought to share 🙂
(btw, I just skimmed the video to see that section; haven’t had time to watch this particular video yet)