Dun is the dilute responsible. A bay horse with one or two dun genes is a bay dun, or a dun. A black horse with one or two dun genes is a grulla.
ETA: Grulla is actually really easy to breed for if you are willing to test your mare and stallion. As long as neither carry agouti, one is homozygous black, and one homozygous dun, then you can guarantee a grulla foal.
Silver CAN and DOES hide on red-based horses. Therefore I personally do not consider it incorrect to whoever calculated based on possibility of silver as the cremello would be red-based and therefore not display any outside signs of silver. Silver affects and often restricts the black pigment but leaves the red be. This is why there is no such outward colour as a 'silver chestnut' but silver bay and silver black crop up fairly often.
It is the same with Agouti. It does not affect the red pigment, only restricts the black pigment, so a chestnut can have Agouti and not show any signs. Mum bred a chestnut stallion to a true-black mare and got a bay foal. It can and does happen.
Not here it isn't! There's actually a lovely filly on the market at the moment that is a silver brown, and she is ConnemaraxWelsh :)
Personally I have never met a silver horse that wasn't a mini but they are around, and at the moment I don't have contact with a huge amount of horses. My riding club and my circle of friends and that's it, because I'm not showing right now. So out of the loop that I don't even know when the next in-hand show is!