This is just something that I like to do. But when you are working with a portrait, and you all know you have the problem of your hand smearing colors all over and all that. Which is annoying as everything. So lay down a napkin or tissue where your hand is going to be sitting. Also, for eyes and teeth. If you do happen to smear or still cant get your white white as they need to be. I recommend before you put any color down, fill in the white spaces. Then if color goes over it, it erases better, and you can just re fill the white. Plus nothing is ever really white, I always have shades of blue grey and lilac in the eye whites, and yellow grey and blue in teeth typically.