As an avid horse girl, I am foaming at the mouth for horses to be added! YAY! But I'm also a geneticist and would love to see some realistic genetics at play if it can be done!
At it's most simple: Have grey horses be a base color at birth and grey as an adult.
Because there is no animal aging system in the game, I think just a grey variant like a snapshot of a real horse's greying progress. For example: three different black based grey horses could still look drastically different as adults. One fleabitten, one dappled grey, one white with dark skin. I'll add pictures of my own horse, Stanley, as he aged! From birth to 12 years old.