Matching a watch to your outfit comes down to two things: matching metals and matching formality. Get those two right and almost any watch will look intentional, no matter what else you're wearing.
Match the Metal, Not the Color
The simplest rule that actually holds up: keep your watch case and bracelet or buckle in the same metal tone as your other accessories. If you're wearing a silver belt buckle or white gold wedding ring, a silver or steel watch case pulls the outfit together instead of introducing a third competing tone. Same goes for gold — a gold-toned watch next to gold cufflinks or a gold buckle looks deliberate, while mixing silver and gold everywhere at once tends to look like an accident rather than a choice, unless you're doing it on purpose as a two-tone piece.
Strap color has more room to move than metal does. Black leather goes with black and dark brown shoes, brown leather goes with brown and tan, and canvas or nylon straps are casual enough to sit outside this rule almost entirely. When in doubt, match your strap to your belt and shoes rather than to your shirt or jacket — leather goods talk to each other more than they talk to fabric.
Match the Formality, Not Just the Look
A watch can look great on paper and still feel wrong for the room. The test isn't just whether it looks nice — it's whether its formality matches the outfit's. A chunky dive watch under a suit cuff fights the tailoring even if the colors are perfect. A slim dress watch with jeans and sneakers can look like you got dressed for the wrong day. Field watches and simple automatics sit in the useful middle ground, which is exactly why they're the easiest first watch to build a wardrobe around — they don't fight jeans, and on a leather strap they don't fight a blazer either.
If you only own one watch, that middle-ground instinct matters more than color-matching ever will. A watch that's honest about being a tool, without trying too hard to look expensive, tends to slide into more situations than one that's dressed to the nines for every occasion.