The subject of this photo has beautiful white teeth, but even the whitest teeth are shaded by the lips, so won't look totally white all over. The scan has darkened the white paper, but you can also see that the lower teeth and back teeth are quite shaded in this drawing. Reserve pure white paper for the very whitest areas of highlight in a photorealist drawing, and with a sketch, suggest some shading where appropriate.
The subject's shiny lipstick makes the lips more defined, but a slightly relaxed approach to the line,and use of shading on the face, keeps them from looking too sharp. The shine of the lipstick makes the highlights very crisp and white.
Note that the corners and inside of the mouth can be quite dark, depending on the fall of light - don't be afraid to draw it that way.
