When to use an underpainting

Paul Lekang  asked on YouTube: "You often, but not always, start by giving the painting an "undercoating." How do you decide IF you should do it ... and how do you decide the color? Does the color shine through in the end - even though you've painted over it (several times?)"

Solar flare underpainting in casein

Paul, you're right, I don't use those undercoatings all the time. I prime a few pages in advance without knowing what painting I may do over them. I use a thin layer of casein or Acryla Gouache, both of which dry with a matte surface that won't reactivate. I use gouache or casein over them, and those opaque colors to shift the color into other realms. 

The priming layer influences all the choices I subsequently make, and that's why I like it. The color of an underpainting inevitably does shine through. Sometimes the underpainting color is similar to the colors I end up using for the finish. 

Other times I use a contrasting set of colors for the final paint. For example, it's exciting when a blue underpainting peeks through an orange painting, or when you glimpse a red underpainting beneath green foliage.