They operate on the principle "what you don't know can't hurt you," they turn completely black and opaque at the first sign of trouble, so you can't see anything that might alarm you.
The color change could also be subtle enough to require a sensor in smartglasses, but I don't think that's too bad. If the lenses ever get to the point where they're relaying multiple things, you're going to need a sensor anyway to demux those signals.