How would I go about adding fingernail polish to a model's nails that are unpolished, and of course, make it look realistic? I'm currently retouching a photo and am stumped at this minor setback. LOL

Any help would be greatly appreciated, and thanks to those that contribute.

It would all be in the shading. Add the color them shape with shading—highlights and shadows.

If there were good strong shading below then I might blend, but remember polish is a thick coat applied above the nail, there is depth to it—thickness.

depends on how the nails look I added nail polish that looks realistic just because the nails had a good sheen to them. I just added a color balance layer set to multiply and then another if the color is too light.

To make the color of the finger nails look realistic...sometimes there are strong highlights on polish and there is some curvature to the nail so it is not all the same color red...the red will be darker on the sides of the nails.  This will give the nails shine and curvature and a more realistic rounded 3d look.

I would consider painting a thin white highlight down the center of the nail to indicate some degree of shinyness.  You can use a low opacity white brush to build up the effect on a separate transparent layer so you can change your mind easily and experiment.

I might also consider using the dodge and burn tools to lighten and darken the nails to get a more 3d rounded look with a highlight and shadow at the edges. 

You could also do this with a curves adjustment layer too.  Paint on the nails with a curves adjustment layer where the curve is pulled up on the higher right side or at the very top right corner to get a white highlight.  The second curve adjustment layer would have the center of the curve pullled down in the center...and you would invert the mask to be black (control or command I to invert the mask) then paint on the the image with the mask selected using a soft low opacity white brush to add the darkening edges to the nails to get a truly rounded look.