Most of the times my creations are poorly color-coordinated; I just use whatever parts I need functionally and don't worry about how it looks. But with the black NXT (in other words, with an option), I started thinking more about how to utilize the color. While I like the white/gray color scheme, the darker pallet had me thinking more along the lines of a zamor shooter... and I already had the mechanism, and enough dark studless parts...
Thus was born Ogre:
The four zamor launchers can fire a little over 9 balls each second, with a large "tray" on the top that feeds about 50 of them and allows for easy reloading. The tank treads have the upper point of the triangle "sprung"... the tension on the tread is maintained by a LEGO shock, so that the treads do not have any slack in them (and when going over a bump, the treads can "stretch" slightly to compensate). It doesn't use any NXT motors (after all, they didn't go with the color scheme), instead driving the treads and the launcher using the PF XL motors. This gives it a nice easy way to run it as well: with the NXT off, Ogre can be remote controlled using two stock PF remotes. But turn on the black NXT, and it can control the entire vehicle using the Hitechnic IRLink, in a fully autonomous mode where it can "hunt" for isolated targets, center on them, and then launch a volley of spheres while advancing towards the target (and disengaging when the target is no longer present). Another fun aspect is a small spy cam mounted to one side. With that, both remote control an dautonomous modes are a lot more fun as you "ride along" on Ogre as it roams across the countryside. Or, well, the living room.
Originally I wasn't sure of the black NXT. And for functionality, after all, a regular NXT does have the same feature set. But after playing around with it, it has given me some ideas for different color schemes, something that has always been rather lacking before: after all, the RCX had just one color (one I liked, but still... only one).