Sterling silver does tarnish over time.
The best way to clean it is with a little soap and water. Dry and polish it with a micro cloth or even a t-shirt. Do not use silver cleaners because they will wipe away the intentional darkening/oxidizing. Wearing it in the shower/bath is a great way to keep it looking new. REMOVE IT when swimming in chlorinated, mineral, or other treated water.
Bronze will tarnish and darken faster than sterling silver.
Bronze ages naturally and develops a rich, dark patina. Depending on personal preference, you might think it's getting too dark and needs to be lightened and polished. One of the simplest ways of cleaning it is by using a little bit of ketchup on a clean cloth. Just gently rub the ketchup with a cloth on the highlighted areas (not the areas that are intentionally dark) and then rinse with warm water and dry completely with a paper towel or cloth. REMOVE it when swimming in chlorinated, mineral, or other treated water.