You can buy cans of hair paint at most costume stores, and that is very temporary. You might need to carry the can around to do touch-ups even. Another trick that we use in theater is to apply white liquid shoe polish using a soft-bristle brush and then use a hard-bristle brush to put little bits of black in it to get a salt-and-pepper look. This second method will stay longer, but it can be really annoying to wash out, and you *need* to wash it out before like sleeping, or the pillow will bond to your head. I kid you not. Hope that helps!
