Van Halen had a clause in their contract not to have Brown M&Ms backstage. This was a tactical play more than a diva move.

The story of Van Halen requiring in their contract rider that no brown M&Ms were served backstage is often cited as one of the most ridiculous requests by a band ever. However, more than Van Halen making a diva move, it was actually a genius move to ensure that they got the quality and level of equipment they needed to put on their show. 

What they really wanted to know is if the concert promoter had read the contract in full. The brown M&M clause was buried in the middle of a bunch of other requests of high-performing lights and sound equipment. David Lee Roth, the lead singer, compared their contract to the Chinese Yellow Pages, because it was so long and complicated. 

Van Halen was one of the first bands to take huge productions into third-level markets. As a result, their shows were prone to a lot of technical errors. If they spotted brown M&Ms backstage, they would know that the rider had not been read properly, and it would signal them to do an inspection of the equipment to make sure nothing would go wrong. 

One time, in a New Mexico concert, he found brown M&Ms, and David Lee Roth trashed his dressing room (about $200 worth of damage, he says). However, because they didn’t read the requirements, their whole staging sank and caused $85,000 of damage! The press caught wind of the rumor that David Lee Roth caused $85,000 because he saw a brown M&M and the rumor caught ever since.

(Sources 1, and 2)