Van Gogh used black to fill in the eyes and added another smudge of black down at the end of the snout for the nose. The bonnet is on the right side of the head, and its purpose was related to it being reported in the papers that the London detectives dressed up like women to try and catch Jack the Ripper in the act.
The bonnet partially covers the police helmet, but the helmet can be seen with its rim on the left side of the head.
Van Gogh added a hand with a knife below the policeman for a little fun. The hand is painted white, and the knife blade is sticking out to the left.
Vincent van Gogh was Jack the Ripper.