Quantity_TOC_RGB: with this system a
color is defined by its quantities of red, green and blue (R-G-B values).
Quantity_TOC_HLS: with this system a
color is defined by its hue angle and its
lightness and saturation values (H-L-S values).
A Quantity_Color object may define a color
from three values R-G-B or H-L-S according
to a given color definition system.