The relative warmth or coolness of the components of a painting represents a powerful but under -utilized tool for building a cohesive image. Many painters choose their colors based on what the source of the image dictates. This approach makes it easy to decide what color to make any part of the painting, but it requires that you spend a lot of time mixing hues that are exact matches. Others tend to base their palette on a collection of favorite colors, always using the same few hues no matter what the source of the image suggests.
While it's true that color choice is a very subjective process, the various aspects of color have a profound impact on the feeling of light, space and volume in a painting, and first on the list is color temperature. When you are deciding what color to make a given shape, it is more important to notice whether it is warm or cool compared to its neighboring shapes than to choose a particular hue.
Emil Kosa Jr.
In this California scene Emil Kosa Jr. has used color temperature to enhance the feeling of light and space. Notice how he changes the coolness of the shadows as he moves further into the distance. He is also adjusting the value relationships, and value plays an important role in creating a convincing illusion, but it is the temperature contrast that accounts for most of the sense of powerful sunlight. Let's see what value does without color:
Even though we can see the value range compress as we move from foreground to background, the space has flattened out a lot without the color, and the sense of a specific place and time of day is gone.
For homework, try putting color into one of the following images, but first, go back and look at the road in Kosa's painting. See how warm he made it in the foreground? Notice the transition in temperature as the road moves back into the distance.