Many years ago I studied with a very well known painter and muralist named Jay Datus. One of the many things I learned from him was the importance of white when doing portraits. He would never use pure white but would rather mix his own white. I have premixed my whites ever since. Squeeze out a generous amount of Titanium White. Add small amounts of raw umber and viridean and mix thoroughly until a cool ivory color is attained. This is an excellent color to use for tinting turning planes or planes in the shadow. Conversely, using the same process mix until a warm ivory color results. Works very well for those planes of warm light.