That happens when you kiss Josy. Your teeth turn to iron
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Wait! Neil, what did you do?

What's the problem with Neil's silver teeth? We know that MCs dad is quite poor. He can't afford the gold ones (it's also very gypsy thing and practice). And even more so, he can't afford an expensive prosthesis, ceramics and similar stuff. So in his position, the choice is not particularly big - to live without teeth or to sparkle with silver ones. And since we see him with such teeth even in Lynette diary scenes, this means that he lost his teeth at a fairly young age. Whatever the reason is, but shit happens. It's definitely not a rendering problem, because he is the only one person in whole game with such teeth.
As for Josy and even MC himself, that's not a rendering problem either. Well-cleaned, literally polished teeth reflect light very well and shine. And the more you bare them, the more this effect can be. While partially covered with lips or with an incomplete smile, this effect can be almost imperceptible. Sit under a light source, smile broadly and look at your teeth in a small mirror. If you are not a heavy smoker or you brush your teeth more than once a week, then you will see glare and even partial reflections in them.
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The brighter the light, the less "metallic" effect is. But when lights is dull or unregular, then teeth can give fancy glare or highlites.
And different models which were used in game have different teeth, their color tone and shiness level too. Just imagine how weird and unnatural it would look if every character will have identical "cloned" teeth. Ok... they mostly are, since we don't see any person with IRL like teeth (sometimes not ideal, sometimes a bit crooked, whatever else, etc). But in such games, such particular imperfections are not what you really want to look at and enjoy.