craigb
It's my understanding that they don't "sprout" but one of the darker hairs that's already grown out loses it's color and becomes gray (or grey if you prefer).
Heh... yeah I figure that's the more likely answer but it's funny to think about some hair folicle playing "The Little Whisker That Could" in the middle of the night as I drool into my pillow.
"Iz is goana be the LONGLIEST and the GREYIEST hair EVAR!!!!"
They still seem to grow faster than other whiskers after they turn "zombie" anyway. Maybe once the folicle doesn't give a fart about producing pigment it can focus on pooping out hair at a faster rate.
/Derp Thoughts with Beeps Herpdy