Emile Gruppe painted in and around Gloucester, Massachusetts in the early and mid 20th century. I saw a lot of his work when I was in Chicago and I really liked his strong color and bold brush work. Later I came to realize that it was his accurate use of values that made his paintings so strong.
While we're drawn to paintings by the use of color, it's the value relationships between the large shapes that make a painting work. When I'm unhappy with the color in my painting, 9 times out of 10 it's the values that are off, not the color. When my values are off in my paining it's usually because I'm looking right at the area I'm trying to mix and the more I stare at the value the worse it gets. When I was told to look at the values next to the value I wanted to mix, then I could get the right value relationship in the painting. This is also true with color.
Gruppe described it as eye fatigue. He has a good paragraph in his book, "Gruppe On Color" that talks about comparing values and color: “You might think that the best way to analyze an area of color is to stare at it intently. But that's just the wrong way to do it. The longer you stare at an area, the grayer it gets. Your eye becomes used to the color; it fatigues; your sense of color dies. The only way to judge color (and value) of an object is to compare it with the color of objects near it.”
Lets say, for example, that you want to determine the color of the sky at the horizon. It can be anything from purple to green. But to see it, you should first look over your head for a few seconds at the color of the zenith. Then quickly lower your eyes. For a few seconds , you will see vivid color near the horizon. Then the color will quickly fade. That's why I constantly move my eyes over a scene, comparing values and colors.” -- Emile Gruppe- Gruppe on Color.