Limited colors on the iphone or ipod make sense. For the most part, it's an item that stays in your pocket. I'm sure some people care about additional design choices, but most people solve this by buying a case (which they'd do anyway).
Beats, due to the fact that they are headphones, are worn as head gear. Yes, they could have stuck with a few simple colors, but it's easier to justify lots of options when people will actually see your user wearing the product. This gives your user an incentive to want a more unique design and be willing to pay for it.