def parse_color(self, color):
        '''
        color : string, eg: '#rrggbb' or 'none'
        (where rr, gg, bb are hex digits from 00 to ff)
        returns a triple of unsigned bytes, eg: (0, 128, 255)
        '''
        if color == 'none':
            return None
        return (
            int(color[1:3], 16),
            int(color[3:5], 16),
            int(color[5:7], 16))