They might have other options in the future, but the muted colors are (1) best for activities—these are touted as hard to dislodge and sweat proof, so great for activities and exercise. This sweat proof material might not be able to be in brighter colors (It's easy to give cake frosting a pretty color, but harder to replicate that color in cookie dough...same might be true of this material). Also, these shades are likely to hide dirt and such if you, say, go on a hike or bike ride. (2) Imagine the white powerbeats in the attached picture in bright pink...wouldn't everyone be staring at your ears? Muted colors mean that they kind of disappear rather than stand out. I think most people would prefer that. Finally (3) Muted colors are easier to match with clothing. You're only going to buy one pair (unless you've got a lot of money to blow), so wouldn't you want a color that could blend with anything? Certain pink colors do go with almost everything...but some can't be worn with anything other than more pink.
This isn't 4D marketing, it's basic marketing. You don't make a complicated piece of technology in a hundred colors—that would cost you a lot, and if the hot pink, and bright yellow and vivid orange pods didn't sell, then you just lost a lot of money on those. So you make the first batch in colors that easily sell, usually black and white (we're lucky they're giving us blue and green also). If sales are good, and the public obviously wants more, and will go for other shades, you add in more colors.