Colors are often separated into warm and cool colors.  Warm colors being red and yellow, with blue being cool.

Warm colors are said to be more active and exciting, with cool colors being more sedate and calming.

Warm colors are said to advance, while cool colors retreat.

This may be due to the personality of the color itself.  A blue being more retiring, sedate, while red and yellow are more active and aggressive.

This may also be due to where the colors are picked up by our eyeballs.  Warmer colors hit further out on the eye, and cooler colors hitting deeper in the eye, thus our physical reception of the color may be the reason why we describe some colors as warm and others as cool.  Of course, it may be the other way around, and we may have evolved to see warmer, hotter, more dangerous colors before the cooler ones, as a safety mechanism.

As we will see later, there is more to warm and cool colors, and the topic is not as settled as it is normally presented.

As artists, we must think not only of warm and cool, but also of weight and intrinsic movement.