The man says, "You can't be Jack and Jill, you're black!"  So, they go off and a while later they come back dressed differently.

They ring the doorbell and once again the man opens the door. "Well now, that is just too darn cute. Who are you this time?"

"We're Hansel and Gretel" says the little boy.

"Well, I hate to disappoint you son, but you can't be Hansel and Gretel because you're black," he replied.

Heads hung low, they leave.  Not too much later the man hears the bell ring again. This time when he opens the door, there stand the two children, but this time they are BUCK NAKED!! "Oh my!”  And just who are you supposed to be now?" he asks. 

"Chocolate M&M's," said the little girl.  "I'm plain.  He's got nuts."