I haven’t worked much with photochromic inks.  That is to say, I should be doing more with these inks.  This technology has been readily available for many years and the effect, while a bit gimmicky, is actually pretty cool.  Photochromic inks react when exposed to concentrated UV light.  In the photos below two photochromic inks were used (red-shade blue and green-shade blue).  These inks appear clear indoors and come to life in sunlight.  As I said, we need to invest some R&D time into this technology as possibilities abound.