Hello Clive: Welcome to our Forum!
Your questions about underpainting are good ones. The thing to know about underpainting is that there are many ways to do it and each way will affect the resulting painting in different ways. Thus, you can study different types of underpainting to achieve differing effects.
Underpainting is not absolutely necessary to oil painting and there are many artists who are accomplished enough to create alla prima with no underpainting.
Many artists, including myself, have done underpainting in monochrome values similar to a black and white photo. The monochrome painting method has been used throughout the ages as not only an underpaininting method, but also a teaching tool. As you have expressed yourself, correct values in painting are a large part of successful painting. Thereby, teachers have students do full paintings in monochrome to teach value and how to effectively create a 3d impression on a 2d surface. The most recognized monochrome painting styles are grisaille (French) and verdaccio (Italian). The grisaille is basically based on neutral or warmish gray tones. The verdaccio is based on cooler gray-green tones. Verdaccio as used in underpainting can really enhance skin tones to create high realism.
Others paint more directly - that is, they underpaint in the same colors they will use in the final painting. Generally, these painters tend to work from the very broad to the very specific and the first layers of paint laid down are their version of underpainting. Many tend to tone/neutralize the colors early on so they end up with an almost dead color rendering of the work as their underpainting.
Still others will underpaint with complement colors as they feel that it can give a vibrancy to the final work.
As you can see, you can spend many years studying differing types of underpainting and the effects they give. For now, since you are trying to learn about value as it relates to painting, I might suggest you try the monochrome underpainting method. Karin's demonstration is a great place to start.
I hope that helps.
