Titanium nitride is gold color, which I would not like.

How about some of the "diamond like" carbon coatings that they use in race car engines? This ends up black and takes on the finish (matte or gloss) of the underlying metal. Very hard, slippery too.

Then there is extra slippery tungsten disulfide, should be easy to keep clean, but it is a silvery color.

I have binocs that have a rubberized coating. Is there some reason why scope sights are not offered with such a coating? I find when I am carrying my rifles around it is more likely the scope that whacks into something rather than anything else. Not so much thinking of protecting the scope as protecting whatever it bangs into.