These color names originated with the X-Windows environment, were adopted by Netscape and Microsoft, then incorporated in CSS2 as a recognized convenience.
The purity of this convention is dubious (i.e., why is "darkgray" lighter than "gray"?); if such things bother you, use RGB or Hex notations instead — e.g., rgb(255,255,255) or #fff or #ffffff.
(or click here for the 216 Browser Safe Colors)