Dark red may be caused by oxidation of the inner surface of the ring or contact with certain chemicals. Here are some suggestions for cleaning and restoring the luster of the ring:

1. Use toothpaste: apply a small amount of toothpaste to the inner surface of the ring, gently wipe it with a soft cloth, and then rinse it with clear water. Toothpaste can help remove the oxide layer and restore the original luster of the ring.

2. Use soapy water: pour some mild soapy water into the container, dip some soapy water with a soft cloth, gently wipe the inner surface of the ring, and then rinse it with clear water.

3. Use alcohol: pour a proper amount of alcohol into the container, dip alcohol in a soft cloth, gently wipe the inner surface of the ring, and then rinse it with water. Pay attention to wearing gloves and glasses and keep ventilated.

If these methods can't solve the problem, consider sending the ring to a professional jewelry store or repair shop for cleaning and polishing.