class TColorGradient: public TColor

TColorGradient

TColorGradient extends basic TColor.
Actually, this is not a simple color, but linear gradient + shadow
for filled area. By inheriting from TColor, gradients can be placed
inside gROOT's list of colors and use it in all TAttXXX descendants
without modifying any existing code.
Shadow, of course, is not a property of any color, and gradient is
not, but this is the best way to add new attributes to filled area
without re-writing all the graphics code.