Coordinating accessories creates a polished, intentional look.
The classic rule is to match your belt leather to your shoes in color and finish. Brown belt with brown shoes, black with black.
Metal tones should also be consistent: a silver belt buckle pairs well with a silver watch, and gold with gold.
In casual outfits, the rules can relax, but harmony still matters. A leather watch strap that echoes your shoes and belt ties the outfit together subtly.