Temperature is another facet of color that adds to the complexity of mixing paint. All colors can be referred to in terms of either cool or warm. The warm shades being yellow, orange, red. . etc. Cool colors are white, blue, green. Yellow is known as the warmest color on ones palette while white is the coolest.
Here's where it gets a little complex. Aside from those two, the temperature of a color is relative - meaning, how you categorize a color depends on the colors around it. For instance, green can be considered a cool color because it has blue in it; however, it also has yellow in it. So if green is next to blue, we would call that green warm because it is warmer in comparison to blue. Compared to red, green is a cool color.
Here's another little twist. Let's take the color red, for instance. If we add yellow to red, making it lean slightly to the orange side, that is a "warm red". But if we add blue to the red, bringing it toward lavender, then we can say that it is a "cool red". There are many subtle shades to any one color, and they all can be categorized as warm or cool.
In the teacup painting above, look at the cool white color in the lower left hand side of the painting. Then compare that to the yellowy color right before you get to the shadow underneath the saucer. It moves from a cool white to a warm white.
In the background, upper right. You will see some yellow mixed in with the blue. The areas where you see more yellow in the blue, that is considered warm blue. Even though blue is just about as cool of a color as you can get, it is described as a warm blue to explain the shade.
What other observations about color temperature do you see?