Font color is important because it allows the text to stand out from the background. If a dark font is combined with a dark background, the font disappears into the background. The same issue occurs when light fonts are used with light backgrounds.
Competing colors also make the text illegible. Combining colors, such as red and green, creates the illusion of the text vibrating on the surface. This causes eyestrain and people would not finish reading the text. Some people (especially men) are color-blind, so would be unable to see particular ranges of red, green and orange. A color-blind individual would perceive the red text on a black background as black.