def text_color(threshold=0.6, formula=:standard)
      brightness(formula) > threshold ? Colorist::Color.new(0x000000) : Colorist::Color.new(0xffffff)
    end
def text_color(threshold=0.6, formula=:standard)
      brightness(formula) > threshold ? Colorist::Color.new(0x000000) : Colorist::Color.new(0xffffff)
    end
Returns an appropriate text color (either black or white) based on
 the brightness of this color. The +threshold+ specifies the brightness
 cutoff point.