What colors does the NXT Color Sensor support?
It depends on which version you're talking about.

NXT 1.0 Light Sensor: This sensor should detect up to 100 shades from white(100) to black(0).

NXT compatible Hitechnic Color Sensor: This sensor will detect 17 colors including white(17) and black(0) or 100 shades from white(100) to black(0).

NXT 2.0 Color Sensor: This sensor can detect 6 colors including white(6) and black(1).

To complete the list (even though it is not backward compatible with NXT) -
EV3 Color Sensor: This sensor can detect 7 colors which are the same as the NXT 2.0 sensor plus “No Color”?  Yes, according to LEGO's website, this new sensor can detect when there's no color.  Does this means white or that the detector is not sensing a color?  This needs to be validated too.

This is one area where, as new versions are coming out, the possibilities of the sensors are diminishing.