Avoid wearing jewellery on your bare skin especially when its hot, sweat coupled with friction on the skin usually reacts with the jewellery causing it to wear and change colour.
To get rid of brassy look:
Soak jewellery in a bowl of warm water, salt and baking powder. Clean off with clean cloth.
Get silver polish (for silver jewellery) and with a cloth, polish the jewellery.
Toothpaste: Squeeze a small amount of toothpaste on a cloth or cotton wool and polish the affected areas.
