 According to Martin Scorsese (qv) on the DVD, when first screening  some test 8mm footage of Robert De Niro (qv) sparring in a ring,  he felt that something was off about the image.  Michael Powell (I) (qv), who at that time had become something of  a mentor and good friend to Scorsese, suggested that it was the  color of the gloves that was throwing them off. Realizing this was  true, Scorsese then decided the movie had to be filmed in black and  white.