Local color is a trap; painters must render the color that actually reaches the eye

Gurney draws a fundamental distinction between local color, the color an object would appear to have under idealized neutral white light, and the actual color it presents in a given scene once atmosphere, surrounding reflected light, and the color of the illuminating light source have all modified it. He argues that novice painters frequently paint from memorized local color, for instance always painting grass a fixed green, rather than observing how that grass actually appears tinted orange in late sunset light or bluish in overcast shade. This substitution produces technically 'correct' but visually flat and unconvincing paintings, because the viewer's eye expects the subtle color shifts that real light conditions produce. Gurney emphasizes training oneself to see past the assumed 'name' of a color toward its actually observed appearance in context, treating every object as a variable rather than a fixed hue. This single shift in observation habit, he argues, does more to improve realism than almost any technical brush skill.

Takeaway: paint the color you actually see under that light, not the color you know the object 'should' be.