Refers to the relative position of a hue on the color wheel. Colors associated with the heat of the sun and fire (yellows, oranges and reds) are considered to be warm; the colors we associate with cool things such as ice and water (mainly blues) are cool. Greens and violets have qualities of both.
A hue’s temperature is relative to other colors. One yellow may be cooler than another, just as some blues or reds are warmer than others. Cadmium lemon is a cool yellow, whereas cadmium yellow pale (or medium) is a warm yellow. Cadmium red light is a warm red and alizarin crimson a cool red. The warmest color on the color wheel is a red-orange. The coldest color is a pure blue (cobalt blue). This helps you determine whether a color is relatively warmer or cooler. Adding white to a color both lightens and cools it.
Note on blues: Blues are a little difficult to classify as warm or cool because the two most commonly used blues, Ultramarine and Phthalo are on opposite sides of the coolest pole, Cobalt blue. Ultramarine blue (blue plus a little red) and Phthalo blue (blue plus a little yellow) are therefore both warmer than a pure blue such as cobalt blue. There are often conflicting views amongst artists on whether Ultramarine blue is cooler or warmer than Phthalo blue, but since red is warmer than yellow, I think there is a case to be made that Ultramarine (which has red in it) is warmer than Phthalo blue (which has yellow, or green, in it). Green is cooler than red.
Website: Gamblin list of color temperatures
Course unit: Observation 2