You are not the murderer, though you are covering for him.  Prince Maroon killed Mr. White, thinking that he was Colonel Mustard.  You aren't sure why he hated the Colonel so much, but you don't care, figuring that your shared secret will tie him to you more closely than ever. 

On the night of the murder, you and Prince Maroon had a secret rendezvous in the barn, agreeing to leave by separate routes.  He went around to the front of the house; you were to wait five minutes and then enter through the conservatory.  However, before you could get there, Maroon burst back out through the dining room in a state of panic.  He had bludgeoned White with a candlestick, which he cleaned and left back where he found it.  The two of you returned to the barn, where you tried to persuade him to run away with you.  He felt that this would only serve as an admission of guilt.  You were still arguing when the body was discovered.