Ok, so I am working on my acrylic/oil painting skills. Up until now I have been practicing simply apply my paint to the surface. Sometimes all go all wild and crazy and tone the ground first. In class today, we started talking about and doing under paintings. I'm really not sure I understand what the concept of the underpainting is for. I was reading a "Painting like the old Masters" book in Barnes and Noble and I see for example, if the artist is doing a nude painting, he/she would underpaint the flesh tone areas grey with all of the values being applied in values of grey. I have seen portraits that are more reddish when completed started with a green underpainting and the painting of a pheasant that I was doing in class was recommended to be underpainted in purple because the bird was burgandy with golden yellow areas as the dominant colors. I don't understand why. Can someone explain it, or at least answer these questions?
1. If an underpainting is done in the compliment of the end point color, how does that help?
2. Doesn't the top layer of paint cover over the underpainting and effectively nullify it?
3. Why would grey be used as the underpaint instead of the compliment.
4. Should only the subject and focal points be underpainted with the designated color or should the entire piece be washed with the color as well.
Help. I'm just not getting it.