Moorit katmoget would be 1. Katmoget and 2. Brown. And would have brown legs.
Grey moufflon is its own single gene and is dominant to the gulmoget pattern. It's expressed at the Agouti locus (set 1 in my first answer to Mallows) and it's possible, though rare, to have both 1. grey moufflon and 2. brown genes, which is what I'm suggesting the lamb has.
A normal grey gulmoget would have 1. gulmoget and 2. black
Here's a grey moufflon moorit, though not a Shetland
They probably just call them 'white', 'dark' and 'patterned' when no-one else is around