All raw brass will deepen in colour over time.
You can use any jewellery polishing cloth or some Brasso or even brighten it with lemon juice.
Raw brass will darken more quickly if exposed to moisture. To keep jewellery untarnished it is advisable to remove it before showering, swimming, applying lotions or perfume.
Gold plating is a process where a layer of gold is placed on to coat the surface of another metal. As the jewellery is not 100% solid gold, extra care should be taken to maximize its lifespan. Anything plated will eventually tarnish with time and wear.