Sweeney Todd is not only consumed by vengeance but goes mad with it, finally destroying himself and taking others down with him. In a way the visual signature of his derangement is his white shock of skunk hair. It’s interesting to note that the last movie character who had the same skunk ‘do was also obsessed by vengeance, so lost in rage that he’s prepared to sacrifice his life, the lives of his crew members and even his ship to find and destroy his nemesis.

Johnny Depp in Sweeney Todd; Gregory Peck as Captian Ahab in John Huston’s Moby Dick

On top of which both men raged and obsessed sometime in the early to mid 19th Century. Has anyone ever tried to make a musical out of Herman Melville‘s Moby Dick?
There’s one more parallel. Both Sweeney Todd and John Huston‘s Moby Dick (1956) both have monochromatic color palettes. In Moby Dick, in fact, Huston and cinematographer Oswald Morris made the first color film with a subdued drained hue, an effect that was achieved in the printing process by blending the color negative with a “grey” (black and white) negative. The idea was to emulate the Currier and Ives etchings of the period.
I only wish I could find a decent color closeup of Peck and his Ahab skunk hair.