Thank you Samantha!
I'm posting the pedigree inline for easier viewing:
Mum shows Cognac, while dad shows Grotto.
The kitten shows Pewter, which is known to be dominant to Grotto.
Pewter must therefore be Mum's hidden eye, and recessive to Cognac.
We're almost there now. Only Odyssey Cognac Moon left to test before Cognac is placed!