You can try Graechan's plastic or glass script-fu on a sample nail.

something like this maybe?
Original
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Edited.
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If you like it, this is what i did.
1) I made a selection to just select the nail, feather selection by 5 px (depends on your image size i guess).
2) use the Smudge tool to smudge from the normal parts of nail toward to tip to get make the tip of nail same color as the rest of the nail and smudged inward from normal parts of nail into the whiter base part of nail to get rid of the white at the base of the nail.
3) Bump the contrast up to like 55
4) Colorized and selected the hue i wanted.
5) Created new layer (Set this new layer mode to Soft-light) and use only pure black to paint to darken parts of nail, and pure white to brighten parts of the nail in whatever weird shape the shiniest spots might be.

Leave the selection in step one throughout all the steps...I cleared the selection when i did this so my step 5 isn't perfect as you can see flaws around the edges...but it should look a lot better if you keep performing work on your selection only.

It's kind'a fun...so i did a 4th nail.
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