They go totally opaque until you calm down.

Feeling murderous: jet black Feeling angry: bright red Feeling calm: light turquoise

This probably isn't that far off. That would keep the contact lenses inert and passive (no added side effects and no power needed).

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.