RGBColor[red, green, blue] is a graphics directive which specifies that graphical objects which follow are to be displayed, if possible, in the color given.
Red, green and blue color intensities outside the range 0 to 1 will be clipped.
On monochrome displays, a gray level based on the average of the color intensities is used.
The end of the Graphics Gallery shows examples of colors generated with various RGBColor specifications.
See The Mathematica Book: Section 2.9.3.
See also: Hue, GrayLevel, CMYKColor, ColorOutput.
Related package: Graphics`Colors`.