Would "slate gray" be too difficult to interpret? We all know slate comes in a variety of colors and could allow that all needed wiggle room by a judge. Or would it allow for the possibility of too much variance between breeders as well? I like the word myself, but have the disadvantage of being able to pick up a grown bird and look at it. Chalkboard slate is the color I think of. My wife thinks of soapstone? Go figure!