code: public ColorReference brighten(final Float brightness) {
  		ColorReference copy = copy();
  		if (copy.brightness == null) {
  			copy.brightness = brightness;
  		} else {
  			copy.brightness += brightness;
  		}
  		return copy;
  	}
docs: Brightens this color by the amount given. Returns a copy of this color
  object not this object itself.
  
  @param brightness
  the amount to brighten (between 0 and 1).
  @return a copy of this color object, brightened by the given amount.