Why is D Gray Man called D Gray Man?

Hoshino called the series D Gray Man because of the color gray. It's not black, and it isn't white. It's in the middle of the two. The black ones are the Noah & Akuma, while the white ones are the humans, and the gray ones are the exorcists.

She went into plus detail about the name in an interview but I can't find the video. Sorry. :/