The purpose of an underpainting is to establish a uniform feeling to the entire painting. This is done by using one color which will show through in parts through the entire piece. It also helps to finalize composition and values before you go in with other colors.
So why choose a color like orange? The color can be difficult to look at especially in large quantities such as this. It also has connotations of being toxic partly because of the color of pesticide. It is also thought of as being unnatural when it is heavily saturated. So that is why I chose it... because it just looks toxic 😷