[_8j50]

They're supposed to display in punycode characters outside of your locale.

Maybe whitelisting specific characters instead of only locale would be best.

Your color idea is neat but look at it from the perspective of people that actually use idn domains and don't speak english. What if раураӏ.com was раураӏ.fans-ùøê.com where ùøê was part of the same charset as ӏ and the user speaks both locale. Even if you color it,it would be difficult to train users to pick up on that.