My first 3DS was Aqua Blue and it did sparkle so, like the jewel that is the ocean.
But then I, with my heart that is as ice, replaced it with an XL model. My XL is Black & Blue because it doesn't know its place and I have to beat the lesson in all the time. No, I'm just kidding, it's Black & Blue because I dropped it down thirteen flights of stairs in one day. No, I'm just kidding, it's Black & Blue because it got mugged by a gang of thugs while it was coming to my house and had its stylus robbed. No, I'm just kidding, it's Black & Blue because it walked into a door, right, honey? No, I'm just kidding, it's Black & Blue because [Insert joke about 3DS being beaten #337 here].
But seriously, my XL is Black & Blue, or as I commonly call it, Blue. The reason? You want a reason? Well, Blue is my all time favourite colour. Ever since the Colympics of '92 (hosted in St. Helens; lovely place, if you ignore the smell), when Blue took home five humans (the Colympics would be confusing if they awarded gold, silver, and bronze; the medals they award are: human (1st), jaguar (2nd), and cassowary (3rd)) he's been my favourite. I know there was the recent 'mixing' scandal but, as the tribunal ruled, he was not turquoise in those pictures, he'd never even held hands with Green, not for a second. Anyway, I chose to honour him by choosing a Blue 3DS XL, so there's your reason.
I went through a fierce mental battle to avoid trading mine in for an XL. I normally would've done it in a heartbeat, because the XL actually seems like a huge improvement over the vanilla 3DS. However, my first 3DS has been the one I've stuck with, just because it's that awesome Zelda themed one that came with Ocarina 3D. It's black, and has the ocarina from OoT and the harp from SS embedded in it, so I just couldn't give something like that up, even if it was for such an appealing alternative. I don't think I'll ever be able to part ways with it, actually.