Color has three attributes: hue, value, and intensity.
Hue: the name given to a color. (ie. green, blue, yellow)

Value: the lightness or darkness of a color. (ie. pale green vs. forest green)
You can change the value of a color by adding white, black, or gray. You can also modify the color by mixing two hues or overlaying two or more hues.

Intensity: the saturation, or purity of a color. (ie. emerald green is more intense than pale green).

The Color Wheel
Twelve hues arranged in a circle form the color wheel. These twelve colors are divided up into three categories: Primary, Secondary, Tertiary.

Primary: Red, Blue, and Yellow. These colors cannot be created by mixing other hues, but all of the other hues can be created by mixing the primaries.

Secondary: Green, Orange, and Violet. Secondary colors are created by mixing two primaries (specifically, the two that are adjacent to the final color). Yellow + Blue (primaries) = Green (secondary).

Tertiary: (Yellow-Green, Orange-Yellow) These colors are created by mixing a primary hue with a secondary hue. (ie. Yellow with Green)