Transition lenses are also known as Photochromics lenses.
What Colors Do Transition Lenses Come In?
These lenses, once exposed to UV radiation (the sun), darken to a light sunglass shade. Once you walk from the outside to inside they also change back to a regular clear lens.
The gray Transition lens goes from a light to a dark once exposed to UV radiation. This lens turns to a gray sunglass shade. The same thing with the brown Transition lens; it turns to a brown sunglass shade.
Which ever Transition lens you choose, gray or brown, you are protecting your eyes from UV radiation and glare.