Here are some notes about color theory, mainly as it applies to art/painting.
Colors, or more exactly hues, can be described as a mixture of two or three primary colors. Different color systems use different primary colors. The choice of primary colors is arbitrary, but most color systems try to choose three colors which are evenly spread along the span of the human-perceptible spectrum.
Light based media, such as CRT displays or LCD projectors, use an additive color system. Such systems typically use combinations of red, green, and blue light (RGB) to produce different hues. In an additive color system, an equal mix of all three primary colors produces white (or, in practice, usually slightly gray). It's useful to think of additive color such that when dim, pure hues are mixed they create colors with added luminosity and neutered hue; mixing additive colors adds luminosity.
Media based on dyes or pigments use a subtractive color system. Historically, painters have used red, yellow, and blue as their primary colors. Technically, however, red, yellow, and blue are not evenly spaced around the color spectrum making the mixing of certain hues—colorful greens, cyans, and magentas—impossible to mix. Red, yellow, blue color systems, therefore, have a relatively small color gamut.
Modern printing systems use the primary colors cyan, magenta, and yellow. Cyan, magenta, and yellow are more evenly spaced around the color spectrum than red, yellow, and blue, so CMY systems can mix a larger gamut of hues.
With subtractive color systems, mixing all the primary colors together in even proportions produces black. Because of imperfections in paint pigments, however, they actually produce a muddy brown. Therefore, most CMY printing systems also include black. It's useful to think of subtractive color meaning that luminous, pure hues are mixed to produce dim, neutral colors; mixing subtractive colors subtracts luminosity.
Mixing together two primary colors produces a secondary color.
red + green = yellow
green + blue = cyan
blue + red = magenta
cyan + magenta = blue
magenta + yellow = red
yellow + cyan = green
Mixing a secondary color with the adjacent primary color produces a tertiary color.
blue + cyan = azure
blue + magenta = violet
red + magenta = rose
red + yellow = orange
green + yellow = chartreuse
green + cyan = spring green
Note that tertiary colors are hardly even described by name. These names are just approximations.
Two complementary colors appearing next to each other appear more vivid. The complement of a primary color is the color you get from mixing the two other primaries. The complement of red is green, the complement of yellow is purple, the complement of blue is orange. (Inversely, the complement of a secondary color is the primary color which was not used to make it--the complement of green is red.)
Adding white produces a tint of a hue, and reduces its intensity. Adding black tends to muddy a color, rather than simply make it a darker tone of the same hue. (Although, adding black to yellow can produce some unique green hues.)
Gamut Roughly, the range of hues that can be expressed within a particular color sytem.
Hue The hue is commonly called the color, like red, blue, or green.
Tone also called value; the lightness or darkness of a color. A monochrome filter would make it easy to compare the tones of different colors.
Saturation also called chroma; the intensity of a color, if it has been diluted with white, or darkened by black or gray. Variations in saturation can be achieved by adding different amounts of neutral gray of the same value as the color.
Temperature hues can be considered warm or cool. Blue and yellow tend to be cool, and red tends to be warm. However, there can be warmer and cooler blues.