returns color darker by step (where step is in range 0..255)

def darker(self, color, step):
        """returns color darker by step (where step is in range 0..255)"""
        hls = colorsys.rgb_to_hls(*self.rgb(color))
        return colorsys.hls_to_rgb(hls[0], hls[1] - step, hls[2])