DefaultColor is an option for graphics functions which specifies the default color to use for lines, points, etc.
The setting for DefaultColor must be a CMYKColor, GrayLevel, Hue or RGBColor directive.
The default setting is DefaultColor->Automatic, which gives a default color complementary to the background specified.
See The Mathematica Book: Section 2.9.3.
See also: Prolog, Background, FontColor, TextStyle.