Van Halen really did have a request for a bowl of M&Ms with "ABSOLUTELY NO BROWN ONES" written into their contract rider. As Snopes details it, though, it wasn't because the band was power-crazed. It was a test, a quick way to see if their instructions had been carried out.
As David Lee Roth says, Van Halen was the first band to bring massive productions to podunk towns, and not every venue could be trusted to fully and safely assemble the extensive stage show that the band brought with them. The presence of brown M&Ms would spur a full check of the entire production to ensure there were no errors.
Sounds eminently reasonable, but Roth doesn't think he needs to explain why he once "trashed the dressing room" and "kicked a hole in the door" when there were brown M&Ms. Was that for safety?