Mirror coating

A mirror is a surface treatment applied over the tint. It reflects a portion of incoming light, which cuts overall brightness on open sunny water without needing a darker base tint. It also stops people seeing your eyes, which matters to precisely nobody who is fishing alone.

Different mirror colors are mostly cosmetic; the base tint underneath does the real work on contrast.

Mirrors scratch. The coating sits on the outermost surface, so a mirrored lens shows wear faster than a plain one, and cleaning it with a shirt tail is how that happens.

There is a second coating worth more than the mirror, and it is on the inside. An anti-reflective layer on the back surface stops light coming from behind you bouncing off the lens into your eye, which is the ghosting you get with the sun over your shoulder.

Hydrophobic and oleophobic coatings are the third kind, shedding water and fingerprints. On a boat in spray they make a real difference.