I was painting a view of the Tucson Mountains when the sun set behind the mountains and the colors went from very warm, definite colors of green, orange and yellow to shades of cool gray color against a very warm, yellow sky. At first the colors seemed grey when the sun set but when my eyes got accustomed to the light change I noticed all kinds of subtle neutral color. With a limited pallette of yellow, red and blue I find it easier to achieve those subtle changes. Find the local color of a shape and decide how much of the remaining primaries you need to add to get the right color, thinking in terms of value first. Large areas of shadow, like a field or mountain, are going to have subtle neutral color changes from front to back and side to side, I find that easiest to control with a limited pallette.