What colour belt are martial arts experts entitled to waer?

Answer 1

black if they really are a master ...

In traditional karate, a black belt is worn by someone at the 1st degree through 6th degree levels. 7th and 8th degrees wear a red and white belt. The top ranking karateka, 9th and 10th degrees, wear a solid red belt.

Answer 2

It depends on the style. In many Japanese and Korean styles black belt is essentially "expert". However some systems have red belts or striped belts to indicate even higher ranks.

Some systems uses sashes, or have no outward rank symbols at all.