If you want to reduce the melanin in the eyes, you could just have the good rest for the eyes. You could use some eye drops with anti-inflammation to clear the eyes. In addition, you could also wear the sunglasses when you are out to protect the eyes from the uv rays and other bad lights. You should keep the good diet to add more vitamin C to moisture the eyes.
Actually, there is no any natural way that can help with reducing melanin in the eyes. Aging can reduce melanin in the eyes during your life. Additionally, surgery has the same work. Many studies have shown that the color of eyes cannot be changed without any help of modern medical treatments.
Frankly speaking, Melanin is the normal dark pigment of the body which gives it its color and sun protection as well. Albions may lack this pigment and have significant health consequences due to that. Some internal organs or color which melanin such as choroid of the eye and meanings of the brain. Melanin producing cells can cause a cancer termed melanoma. So I do not think it is a good idea for you to reduce it.
On the one hand, you can wear light-colored contact lenses if you want to let the color of eyes become bright at any time. On the other hand, you can eat some food which can reduce the melanin and use tonic cosmetics. But it need a long time to achieve objective, i think the most important factor is to keep it up.