>>> idx.drop('green', level='color')
        MultiIndex([(0, 'purple'),
                    (1, 'purple'),
                    (2, 'purple')],
                   names=['number', 'color'])

        >>> idx.drop([1, 2], level=0)
        MultiIndex([(0,  'green'),
                    (0, 'purple')],
                   names=['number', 'color'])
        Nrg