I’ve just read one of American Cinematographer’s great collection of interviews about the making of the Star Wars films: (https://theasc.com/magazine/starwars/articles/starwars/mm/pg3.htm)

This one is about the motion-control model photography, and I’m a little confused by this statement.

the star-move is photographed directly onto color negative and set aside for laboratory delivery

We know they had black and white film stock on hand, because the same interview discusses them shooting motion-control elements on b/w to check they all line up.

So why would the monochrome star fields be shot on colour negative? Is there some inherent difficulty in optically compositing colour and b/w film together?