As a general rule of thumb, it is good to be consistent with metals on your jewellery. If you have a yellow-gold wedding band, best to stick with yellow or rose-gold (rather than silver or platinum) for the case of the watch. Exceptions to this include steel sports watches and small hardware on jewellery – for example the clasp on a bracelet.
The most important question for most sartorial men will be the colour of leather strap that is acceptable for different outfits. Let’s use the leather of the shoes in the outfit as a proxy for formality.
Black shoes: A black strap is obviously the easiest choice, but anything else that is either very dark or very pale can work. For example, a navy-blue (as seen on my Cartier) or dark green, and at the other end of the scale, a pale tan.
The ostrich strap on my JLC (above) is just about pale enough to work with black shoes. Any darker, and it would be too similar to the next leather choice, brown. In exactly the same way as you wouldn’t wear a dark brown belt with black shoes, a dark brown watch strap looks like a mistake.
Brown shoes: Pretty much any shade of brown strap will work with brown shoes. And unlike a belt, you don’t have to worry about the two leathers appearing similar but slightly different, or about mixing skins (calf/alligator).
Everyone knows watches have got bigger in recent years. I express no opinion on whether they look good or bad, but bear two things in mind: a larger watch is usually less formal, and therefore less suited to a suit; and your watch should really fit under your shirt cuff. Those two things should deter most men from buying oversized watches.
Given that point about formality, a larger watch is better suited to casual clothing and sporting activities. I normally wear my IWC Portuguese or Rolex GMT with jeans and knitwear, for example.
Do keep in mind that a smaller watch in such situations can be a nice change of pace, however, and often appears more sophisticated than a big rubber chronograph. A NATO strap is also a nice alternative with a steel watch.
Speaking of steel sports watches, I understand why many men buy them as their only watch. They can seem more versatile than a leather option, good with both suit and jeans. But unless small (and probably vintage) they rarely compliment formal clothing. I would suggest to such men that a leather-strapped, modestly-sized sports watch would be a better choice. A diving watch can be the next investment – in two or three years’ time.