def to_grayscale(formula=:w3c)
      b = brightness(formula)
      Color.from_rgb(255 * b, 255 * b, 255 * b)
    end
def to_grayscale(formula=:w3c)
      b = brightness(formula)
      Color.from_rgb(255 * b, 255 * b, 255 * b)
    end
Converts the current color to grayscale using the brightness
 formula provided. See #brightness for a description of the
 available formulas.