This subject was fairly foreign to me when I started painting. I mean, I knew snow was cold and that blue usually meant cool, but it wasn't always clear to me why, nor how to utilize the knowledge to make a painting sing. And it's not only the hue of paint, but also the temperature of the light source in the painting. For example, the sunlight in Mexico was very warm (gah! this is confusing! By warm I do not mean it was making me sweat…I mean all the colors were saturated with warmth because of that big beautiful sun) A cloudy, winter day's light source is cool in comparison and will cool all the colors of your subject.
In Painting 101, we learn that cool colors recede and warm colors come forward. Since white (think clouds, snow) is cool, then making a background go back requires adding white to the color mixture. Look at a painting of mountains and see how the ranges are increasingly bluer and lighter as they fade into distance. Doesn't it give a 3-dimensional illusion? Doesn't it satisfy something in your mind about the authenticity of the subject?
But, like values, temperature is interpreted in the relationship of colors next to each other. A warm orange is going to really heat up next to a cool blue. A yellow that leans toward green will look even greener and cooler with a warm, orangey red next to it. So, once again, every brush stroke of paint requires the artist to be engaged, thinking, making informed decisions. (I am preaching to myself here!)
What happens when an unfortunate decision in color temperature is made; for example, a warm orange hillside behind a red barn? The viewer's brain becomes confused about what the focal point is and where the eye should travel through the scene and will probably lose interest and, alas!, wander off to the next painting in the gallery.