Warm and Cool Colors
Warm and cool colors are important in painting for a number of reasons. The most obvious is that warm colors tend to move forward in space, while cooler colors tend to retreat. This is mainly because of how we experience the world and is pure science. Within the atmosphere there are zillions of little water molecules floating around. The largest of which form clouds and eventually dump rain on us. But the air itself also contains water. So when we are looking faw away to the horizon we are actually looking through a lot of air, which in turn means we are looking at more and more water molecules, and the hills in the distance start looking blue and purple. This is called atmospheric perspective. And it was first labeled and widely used by none other than Leonardo Da Vinci. The second feature of atmospheric perspective is that objects also tend to lose their contrast and take on the color of their background (ie. sky)
Mona Lisa (Leonardo Da Vinci) Classic example of how cool colors recede and warmer colors come to the forefront.
The second thing to remember about warm and cool colors is how they create balance and a sense of fullness within a piece. This is easier to do when painting a landscape where there could be a cool blue sky, and this could be contrasted against the warm orange color of some leaves. But take a close look at an object, and you should be able to see a multitude of other shades and colors within that object itself. The cooler colors can be tints, shades, or tones. But be careful because you can also find warmth in certain shadows as well. However, generally cooler colors are used as shadows, and warmer colors as mid tones, and highlights.
In this painting by Lucien Freud we can see very cool colors dominating many of the shadow tones. This may be because Freud often lit his models with fluorescent lights. Nonetheless look at the nice interplay between warm colors and cool colors in this piece. I've made a small scale of colors taken directly from this painting as an example to further study.
Extra Credit: Create a small painting using these same warm and cool colors for all of your values.