This is the system that you probably learned in grade school, using poster paints or crayons. It is included here for the sake of comparison, since many people will remember it and perhaps wonder why the colors we use on computers are different. It is still valid for paints, although artist's colors generally are selected from color guides, using generic or proprietary names such as “cadmium red,” “burnt umber,” “thalo blue,” and so on. These colors could be described (approximately) by most of the systems that follow (HSL, RGB, etc.), but they usually aren’t.
red, yellow, blue
orange = red + yellow,
green = yellow + blue,
purple = blue + red.
red + yellow + blue = muddy brown.
Black and white are separate colors, which may be mixed with others to make lighter or darker shades or tones.