“When you consider the light in your painting think of it as something separate from form and color, although light causes both.” John Carlson, chapter 5, Guide to Landscape Painting. Carlson, early 20th century landscape painter and author of John Carlson's Guide to Landscape Painting, is saying that light is something other then local color.
If I’m painting a red barn on a sunlit day then I’m really painting the effects of sunlight on the barn. Sunlight is a warm color and shadows, on a sunlit day are a cool color. So, the red barn turns to a lighter reddish-orange in the sunlight and in the shadow it turns into a red-violet.
The local color of the barn is red but we don’t really see it on a sunny day, instead we see the effects of the sunlight and shadow.
The only time we really see local color is on a cloudy day. Color is a lot richer when it's cloudy. There isn't any sunlight effecting the color and value. We tend to think of cloudy days as gray in color but they are actually more saturated. So when you're mixing color, remember you're mixing the effects of light.