Hmmm.. I've been thinking about this a lot!
With Apple's #1 flagship device now shipping in Gold. What about the iPads? I did a quick search on MR, and this thread came up..
I think a device made my Apple really has to mature, or really be a suited product to come in colours, otherwise they won't do it at all.
The reason the transparent-Mac was in colours is because this gave the device a more premium feel than just plastic.
iPod speaks for itself.
But iPad? iPad with Retina display is a staple business device, a great consumer device and a device hitting the educated system hard. But 1 or maybe 2 or these isn't appropriate for colours.
iPad mini? It's by far a "funner" device than the standard iPad due to its size. But a palette of colours? I think it's too soon.
Gold, Silver and Space Grey for both I think.