As a cross-reference, see W S Taylor's copy of Balthazar from Burne-Jones's watercolour of 'The Star of Bethlehem', squared up for transfer for tapestry design. Taylor drew Balthazar, as well as other heads from the original watercolour, by tracing Burne-Jones's original design, then colouring the design to contrast the king's dark skin with the pale linen hood. It should be noted, however, that Taylor did not trace from Birmingham's version of 'The Star of Bethlehem', but from a second, unfinished version also done in watercolour (with gouache and gold as well) from 1887-1888, now in the collection of Lord Lloyd-Webber. At the time Taylor would have traced it, the second watercolour was in the possession of the artist's family.
See also Lloyd-Webber's full length study of Balthazar.