'I asked if he had any particular mode or rule for mixing his colors: He said, 'No, I mix them as I put sugar in my tea; according to my taste. The whole theory of shadow may be taught by a billiard ball--the simplest object I can think of. Lay it on a table and draw it. You first sketch a circle; you then look at it, and see there is one light, one shadow, and one transparent reflection; on the gradation of these all painting depends. My rule for a portrait is, one-third light, one third dark, and one-third demi-tint.'
More from the same letter;
"The rest of the family were satisfied with the portrait painted in 1806, but I thought there ought to be another, at the age of fifty-two years. My father (Josiah Qunicy) complied with my request, promised to give me the portrait, and in November, 1824, I accompanied him to the house of Mr. Stuart, in Essex street, Boston. ... His canvas was ready on his easel, a bold outline was sketched in chalk, and while conversing rapidly, Mr. Stuart began to put on his colors apparently at random, but of course every touch told. Presently a bright shade of blue appeared in the upper part of the canvas, and Mr. Stuart said to me, 'Your father is an active man, and likes to be in the open air ; he shall have blue sky behind his head. Few artists would attempt to give effect to a portrait with such a light background. It is a bold effort; but I will try it."
~from The Life and Works of Gilbert Stuart by George Mason, Charles Scribner's Sons, New York, 1894, p 243