Black and brown are the genes at a specific place (there are three places for different fleece genes)
Black is dominant over brown.
Two brown genes and a 'plain' gene at a different place, gives you a plain moorit.
Two brown genes and a katmoget gene at the different place, gives you a fawn or a moorit katmoget (there is a modifier gene that alters the brown slightly). A fawn katmoget has a more 'ginger' colour and the moorit katmoget is a bit darker brown.
At least one black gene gives a black fleece, but with a katmoget gene too, gives a grey katmoget.
So they are not called brown and black katmogets, they are called fawn/moorit katmoget and grey katmoget.
Shetland Sheep geneticsI find this useful.