In many forms of art, colour plays an important role in visual composition. Basic familiarity with colour theory is therefore essential to art appreciation.
Colour has three fundamental properties: hue (e.g. blue, green, red), saturation (intense vs. pale), and value (light vs. dark).
Hue can be described in terms of temperature. Warm colours (red, orange, yellow) have an active, exciting appearance and seem to "advance" toward the viewer. Cool colours (purple, blue, green), on the other hand, have a stable, calming appearance and seem to "recede" from the viewer.
The three primary colours (in standard art colour theory) are red, yellow, and blue. Combining pairs of these colours yields the three secondary colours: orange, green, and purple. Hues can also be grouped into pairs of complementary colours (i.e. "opposite" colours), including red-green, yellow-purple, and blue-orange.
Saturation is the "intensity" of a colour. As the saturation of a given colour decreases, it looks increasingly pale or dull. A pure colour has 100% saturation, while earthy colours (aka "earth tones") feature mid to low saturation.
A colour's value is the degree of its lightness/darkness. Decreasing a colour's value to zero results in black. Colours described as pastel are high in value but low in saturation (i.e. light pale colours). Placing a dark colour next to a light colour emphasizes the value of both colours: darks appear darker, and lights appear lighter.