Silver, brass, and bronze have their natural characteristic of tarnishing.
Therefore it’s a natural case if the jewelry color change after long time.

But you don’t need to be worry, because you can do several simple steps to handle:

1. Prepare small bowl, dishwasher soap, soft tooth brush, and warm water.
2. Mix the dishwasher soap with warm water on the bowl.
3. Soak the jewelry in the mixture of soap and warm water in 30 minutes.
4. Brush gently the jewelry.
5. Wash the jewelry with clean water.
6. Dry the jewelry with soft dry cloth.  
7. If you have silver polish liquid, you can just apply the silver polish to soft dry cloth and rub the jewelry.  
8. Your jewelry is ready to wear again.

Tips to keep the jewelry stay shine:

1. Always remove your jewelry before  bathing, showering or swimming.
2. Do not expose jewelry perfumes, lotion, aerosol spray.
3. Do not expose jewelry to extreme temperature / humidity.
4. Do not expose leather to water / humidity or any kind of non-leather cleaning liquid.
5. Wrap each piece individually with tissue/ cloth/ plastic bags, or cloth pouch to avoid scratching.
6. Give humidity absorber inside your jewelry box/pouch.