All the different root tip colors really add to the beauty of the plants, don't they? I grow most of my Neos at a South window, so the new roots are just at the cusp of breaking out (as opposed to the ones in the orchidarium which are all well on their way) so it's really fun to start looking for new roots.
I think lambelkip is right about differing light levels being the cause of different root tip colors. My neo's normally got pink tips, but there are a couple roots, just barely visible, growing down into the moss, and they definitely have light-green tips. I suppose it could be higher exposure to moisture as well, since it's down in the moss, that keeps them green.