1. Colours used
It is really worth experimenting to find out what various consequences may be created by utilizing various colours and colour combinations for the underpainting. Many pick a lot of different colours as a kind of template for levels to be painted on top.
Although grey is primarily used, browns may be utilized as well.
Verdaccio is the title given to a certain combination of pigments. Those pigments are yellow, white, and black. This particular combination of pigments was quite generally employed by Italian artists for underpainting.