Right now, the effect is individual hairs, and it looks great. If you add another color and start criss-crossing the black, you could lose that effect. Doodle on some scrap and you'll see what I mean.
I can only think of one way not to cross - using a twin needle. While it would work, you'd have to move slooooowly, and stop with your needle in the up position when you change direction. I'd save the white for the blue-eyed cat.
It is wonderful!!!!!