Because white and black are not really colors, they are said to represent achromatic contrast. Black and white also represent the highest level of contrast. Complementary colors from the color wheel represent high chromatic contrast. The human eye is more sensitive to achromatic contrast than it is to chromatic contrast, which is why the .jpg compression algorithm will drop color information before it drops black and white information from an image as you increase the compression factor. Monochromatic contrast ramp using blue A monochromatic contrast is set up when you use a single color, then increase or decrease its lightness.
Contrast is extremely important in Web design.
And there are many different ways to use it, all of them based on the other color concepts listed above. It is clear to everybody that black text on a white background is extremely easy to see, which is part of the reason why most of the printed material you read is black text on a white background. Likewise, white on black produces high contrast, but it is more difficult to read because black is perceived as being heavier than white and, thus, squeezes it out a little. The same sort of effect happens with warm and cool colors. The warm colors are perceived as advancing out of the screen slightly, and the cool colors recede.