If the hand turns black after using the nail polish light, the following measures can be taken to restore it:

Avoid continuing to use nail polish lamps: Firstly, avoid continuing to use nail polish lamps, as ultraviolet rays can accelerate the generation of melanin in the hands' skin.

Apply sunscreen: Before coming into contact with nail polish lights, you can apply some sunscreen to reduce the damage of ultraviolet rays to the skin.

Eat foods rich in vitamin C: Vitamin C can inhibit the formation of melanin, so you can eat foods rich in vitamin C, such as citrus, strawberries, vegetables, etc.

Apply whitening facial mask: some facial mask with whitening effect can be used to help fade melanin.

Avoid staying up late: Staying up late can slow down the body's metabolism and easily form melanin, so it is important to ensure sufficient sleep.

Avoid exposure to sunlight: Exposure to sunlight can cause melanin deposition on the skin, so try to avoid going out in strong sunlight. If you need to go out, you can use a sunshade or wear long sleeved clothes.

Persist in washing hands with rice washing water: Washing hands with rice washing water can remove dirt and grease from the hands, and also whiten the skin of the hands.

In short, to restore the color of hand skin, various measures need to be taken, including avoiding the use of nail lamps, applying sunscreen, eating more food rich in vitamin C, applying whitening facial mask, avoiding staying up late, avoiding exposure to the sun, and insisting on washing hands with rice washing water.