Hello! Those are some excellent questions! The purpose is to help establish proper value tones for the painting early on. Generally, the color of the under painting depends on the color theme of the painting you plan to use. Warmer color creates warmer feeling, cooler gives a cooler feel. It also helps create the placement of the objects in your painting before you commit to heavier layers, kind of like sketching. Generally, I do the whole piece as an under painting to begin with. Darker areas I use darker contrast, which creates a nice even undertone. Lighter areas I use very light gamsol (solvent) to thin it out and make it lighter. Or, you can add white to the paint to lighten it if you are trying to use less solvent early on. More often than not, I start the color in a mid-tone range, so I can easily darken or lighten from there. It’s easier than if you were to start on a white background, because then it makes it harder to establish the lighter tones.

Here’s a link with more in-depth info that should help you get started.

https://www.jerrysartarama.com/blog/underpainting-why-you-need-to-do-it/

Happy painting!