Dougal and the blue cat (1970)

A blue cat named Buxton is found in the Magic Garden. With help from the Blue voice (played by Fenella Fielding), Buxton enters the ruins of the old treacle factory where he is crowned king after correctly identifying the colours of seven doors (coloured different shades of blue). Buxton throws all the characters of The Magic Roundabout except Dougal into prison and steals Zebedee's magic moustache.