IMO, formal (suit) and black tie (tux) are really only appropriate with a dress watch. James Bond can pull off a diver with either, but I'm not James Bond. For a dress watch, I think a black/grey or silver/white dial is universal for whatever color you may be wearing. You'll hear some people say that Arabic numbers have no place on a dress watch, that simple hour markers or roman numerals are only acceptable, but I don't know if I buy into that. More important is to follow the general fashion advice of matching metals and leathers: If your belt buckle is silver and your shoes are black, your watch should be silver with a black leather strap. For business casual, modern casual, or smart casual, I think anything goes as far as watches. I don't know that I would pair a smart watch or a rubber strap with a blazer or sport coat, but that may depend on the situation. Anything that fits the style you're trying to assemble is appropriate. Casual is go for anything except a dress watch.