All possible colors can only be organised in three dimensions.
    So they form a cube.
    One corner is black, the opposite one is white. 
    All greys are on the central axis, having equal amounts of red, 
    green and blue.
    Starting on the black side the three corners are red, green, 
    blue. The other three are yellow ( red+green ), 
    cyan ( green+blue ), magenta ( blue+red ).
    What you see here are slices of the cube, from black to white.
    Actually they should have nice hexagonal forms.
    The numbers: on top is the amount of green, low-left is red, 
    low-right is blue. In html it is #rrggbb, so don't get confused.
    ( c = 12, f = 15 ) /Jasper, mai 2005.