I would think that a shade that could be described as grey would be considered silver: like gold, a shade for solemn Masses. Like when gold is used, the dominant color of the chasuble would be in the trim. Just like a gold chasuble chiefly trimmed in red is a red one, chiefly trimmed in violet is a violet one, chiefly trimmed in green is a green one, and so on, I would think a grey chasuble chiefly trimmed in red is a red one. Otherwise, I’d think grey would count as a shade of black.
Having said that, in a case like this, what the vestment maker had in mind and what the priest wearing the chasuble thinks cannot be safely assumed to be the same thing. A person does not have to be licensed in knowledge of liturgical colors to make a chasuble and stole, after all. Unfortunately, the dominant color of a beatiful vestment can be a bit too ambiguous to be practical.