This article is from the Korats Breed FAQ, by Dennis Ganoe firstname.lastname@example.org with numerous contributions by others.
The Russian Blue and the Korat share a great many words in common.
Both are described as medium, silver-blue shorthair cats with
green eyes. A Korat is generally heavier for the same size cat
than a Russian Blue. Korats are stockier and a bit chunkier than
Russian Blues and the Korat has rounded lines where Russian Blues
have longer and straighter lines. The Korat has a single coat and
the Russian Blue has dense double coat. The Korat is gun-metal
blue which is darker than the usual Russian Blue color. Both
breeds have silver tipping on each hair. The Korat's eyes are a
peridot green and the Russian Blue has emerald green eyes. The
head structures on the two breeds are distinctly different and the
personalities are very different.