According to this, the diabolical pact had to be written with blood (human or animal) or red ink, and it was believed that books on black magic were written with a red ink which colour was so intense that blinded any person that was not familiar to that art.
Satan and other demons were often depicted as black men, and/or riding a black horse, and dressed in black or red. It was said that black animals were sacrificed to them. When demons appeared in the shape of animals, generally they were black.
Nonetheless, sometimes demons were depicted riding pale horses, perhaps due to a folkloric tradition that associated the pale horse with the Dead (as a character). Despite all this, some demons were depicted riding other animals and dressed in different ways.
The association with red and black possibly derives from the idea of the red fire of Hell, a place of darkness. The association with red might be due to an exegesis of the Book of Revelation 12:3-9, referring to a red dragon compared with Satan. Black, the colour of darkness, might also be due to many allusions to Hell in Matthew's Gospel.
Many representations of the Devil depict him with red skin.
Nicholas Remy cited that Pythagoras mentioned black as the colour of evil, and thus the animals or other things sacrificed or offered to the Devil had to be black.
See also Nature of Hell.