that's not the problem...if it was a PAL/NTSC incompatibility, NOTHING would show up.
i'm more inclined to believe that one of your component cables (if you are using them) slipped out. i think that if blue is out, the result is black and white?
or
i have a 5 in 1 A/V pack for the xbox that includes all of the standard connections plus high definition and optical audio, as well as s-video. when that pack is set in the wrong mode, it displays black and white (standard definition switch + high definition cables or vice versa)
or
if you are using the standard A/V cable, try getting a new cable, or using a friends.
a little more info on your setup would be nice too, i.e. chip / A/V cable / could be a TV problem?
do other TV sources show up fine?
-jeesusfreek