Silver, silver-plated, brass, gold-plated, or gold-plated jewelry tends to darken or yellow.
This phenomenon is normal and is due to oxidation.
The easiest way is to use a chamois or velor cloth and lightly rub the jewel.
925 silver jewelry can be cleaned with a little toothpaste.
For polymer clay and fabric jewelry, 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 with your purchase or in a closed box, and keep it out of direct sunlight.
Do not use commercial cleaning products.