Underpainting is a technique where colours are applied to a model to form a base for later paints. The purpose of doing this is to use the underpainting to affect the colour value of later paints, and increase their lightness. Lightness in this context is the perception of luminance by the human eye, and there’s a whole bunch of science behind that because the human perception of light is non-linear compared to the brightness of an object, so something twice as bright is not perceived by the human eye as being twice as bright. (Click on the link for more…)
https://www.goonhammer.com/how-to-paint-everything-underpainting/
