http://revach.net/article.php?id=2497

Section: Halacha   Category: Tshuvos
Can You Wear PhotoGrey Glasses on Shabbos? Can You Draw On A Mirror?
The Mishna Brura (320:59) says that coloring on Shabbos is only assur if the color will remain permanently, but if you color something that will disappear after a short period of time you are Patur.  The Shaar HaTzion says that although you are patur it is still assur.
 
What about wearing Photogrey glasses which turns into sunglasses when exposed to the sun and lose their color when out of the sun?  The Piskei Tshuvos says that Igros Moshe, Btzel HaChochma, Shevet HaLevi, Mishneh Halachos and others all say that it is permissible.
 
The reason is that this is not considered coloring.  When the sun reflects off the lens, it appears colored but nothing actually changed in the glass.  This is similar to a mirror where by standing in front of the mirror you can change the image on the mirror but it is permissible since you haven't done anything to the mirror itself.
 
Important Note: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.