There must be many paths to so great a truth.
A lot of talk lately on various underpainting techinques, and some controversy over what is perceived as "right" vs. what is "wrong".
I don't know if you can really define very many ways of painting or underpainting as "wrong".
After all, with the advent of new materials, there are very few hard and fast rules regarding the specific techniques used to properly build a painting that still apply in all situations. As an example, we all hear a lot about Fat Over Lean. Well, if you use a drier in your paint that rule, for the most part, goes right out the window. We won't argue the merits or problems of driers here, ok?
So, while I underpaint rather thin, Karin Wells underpaints rather thickly. Does that mean that one of us is right and the other wrong? No, it does not. It means that we work differently to achieve our aim - a good painting. Karin's way works for her and my way works for me - you have to determine your own way of working too. Heck, some people don't do any underpainting at all!
I have done a lot of underpainting in verdaccio and grisaille. I really like that method, and recently I was introduced to a more direct method by Bill Whitaker which I like too. Bill's way works for him and I found that it worked well for me too - but there are many, many ways of accomplishing a good painting. I really believe that you can underpaint in full color, in monochromes of brown, gray, green, blue, or any color in the rainbow - as long as it helps you to produce a good painting.
The only "truth" about underpainting I know is this:
You underpaint to establish a baseline for your painting which gives you the benefit of removing a series of decisions you don't want to be making in the color stages of the painting.
I can post the progression of my underpainting technique that I learned from Bill Whitaker as another example of the very wide variety of ways of working. It is much harder to say - "here is the underpainting" as the painting sort of progresses.
BTW: This is a studio piece and is from a model, but is not a portrait commission. Also, it's a bit fantastical, so be forewarned.