Temperature is the concept of relative "warmth" or "coolness" of colors. In general, colors in the red-yellow range are considered warm and colors in the green-violet range are considered cool.
However, this definition of “temperature” doesn’t take into account the different temperatures within a color range. There can be warm and cool yellows, warm and cool reds and warm and cool blues. There can even be warm and cool purples, oranges, and greens. It’s a relative concept. Even grays can be warm or cool, depending on their blueness or brownness. The determination of whether a color appears warm or cool is relative. Any color can be made to appear warm or cool by its context with other colors.
Some have attributed psychological effects to color. It has been theorized that warm colors are more active and cool colors are more relaxing. I remember once painting a room that I worked in a fairly intense blue, thinking it would be pleasant. In fact, it proved to be much too aggressive and grating for my liking. For artists, most pigments and papers have a cool or warm cast, as the human eye can detect even a minute amount of saturation. Gray mixed with yellow, orange or red is a "warm gray". Green, blue, or purple, create "cool grays".
An important factor about color temperature is the apparent “depth” of the color on a picture plane. For instance, in a landscape, warm colors will tend to signal closeness and cool colors distance. You can’t apply this idea rotely however. There are many factors that signal depth on the picture plane, as will be discussed on the chapter on Depth. A student once asked me, for instance, “if red advances and green recedes, why does it work to put a small red house in the midst of a green landscape?” Other factors than color are at work here: color intensity and saturation, level of different elements on the picture place, our “expectation” about the picture. So color temperature is just one actor in the play.
It’s instructive to do a color wheel with emphasis on the warm and cool colors. Place the tube colors you own on the color wheel and notice the warm and cool aspects. For instance, the yellow-orange-red side of the wheel is obviously warm, and the purple-blue-green side is cool. But how do colors interact with each other? Can you notice which are the warm and cool yellows? warm and cool reds, warm and cool blues?
Do a “monochromatic” painting, using analogous colors, such as red- to blue-purple, cool to warm reds, blue- to warm greens. Choose a particular bracket of color, such as blue or yellow, and within that bracket, move the paint mixture from warm to cool. Picasso’s “Blue Period” is an example.
Do a still life but limit the color scheme used in the painting to either a cool or warm feeling. In other words, use the complete palette of color but mix colors that are only cool or only warm. Even the neutrals should be cool or warm (i.e., blue-grey –cool– or brown-grey –warm.)