Primary colors
Primary colors are not a physical but rather a biological concept, based on the physiological response of the human eye to light. The human eye contains receptors called cones which normally respond to red, green, and blue light. All other colours can be created by mixing these three colours.
Secondary Colors
The secondary colours are what you get when you mix any two adjacent primary colours. Red and green give yellow, red and blue give you magenta and a mix of green and blue result in a cyan colour.
Tertiary Colors
To complete the colour wheel we need to add the tertiary colours. The tertiary colours are those which lie in between the primary and secondary colours. As you can see on the colour wheel, they are a further blending of adjacent colours.
Color Wheel
Adding it all together, we get the complete colour wheel. Color Wheel - A circular chart of colors from which many color relationships can be derived. Think of a color wheel as a map for locating colors. The route you travel between the colors is what you use to create your color scheme. Some of the most appealing sites follow explicit rules, or paths, in order to find colors that go well together.
Other colors
Every other color available for use on the web is a variation in tone, tint, or shade. Combining natural hues with the neutral colors black and white (and the grays in between), gives these variations.