With veneers and crowns made of porcelain they are very color stable and will not change color. If your dentist makes them VERY see thru then over time the tooth behind them can darken and thus make the porcelain appear darker. But to answer your question...No porcelain does not lighten ever....
Do Porcelain Become Lighter in Color with Time ?
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Porcelain veneers do not change color
You can expect the color of your Porcelain Veneers to remain the same for the rest of your life. The exception is is the Porcelain material chosen is very translucent and the tooth underneath darkens in time, the overall result will be about a half of a shade darker.
Porcelain veneers are extremely color stable
You should plan on the color of the veneers being "what you see is what you get" and not plan on any color change. Normally, if there IS a color shift, it is by becoming darker (more yellow), but that is the exception and not the rule.
One of the great benefits of porcelain veneers is that they won't change color. This CAN be a problem if one desires to bleach their teeth later, as the veneers WON'T bleach.