http://www.rottentomatoes.com/m/1006207-dr_jekyll_and_mr_hyde/

Completely deranged and brilliant pre-code version of Stevenson’s famous tale, using ground breaking (at the time) special effects.
The secret of the transformation scenes was not revealed for decades. Mamoulian himself revealed it in a volume of interviews with Hollywood directors called The Celluloid Muse. Make-up was applied in contrasting colors. A series of colored filters that matched the make-up was then used which enabled the make-up to be gradually exposed or made invisible. The change in color was not visible on the black-and-white film.

https://www.youtube.com/watch?v=wbg5oXpq42Y