Silver, silver plated, brass, gold or gold plated jewelry tends to blacken or yellow.
This phenomenon is normal and it is due to oxidation.
The simplest is to use a cloth in chamois or velvet and to rub the jewel lightly.
For jewelry made of polymer clay and fabric, it is advisable not to put them in contact with water, perfumes, beauty products and not to sleep with them.
Remember to store your jewelry in the cotton pouch offered during your purchase or a closed box, and keep it out of the sun.