If i overdose on vitamin c will my melanin level decrease(especially my eye colour)

what factors in diet and supplements cause your melanin to decrease

You can't overdose on Vitamin C, it is a water-soluble vitamin so you just pee out all the excess. It does not affect your melanin either.
I don't understand what you mean by causing your melanin to decrease? Do you mean you want to be more light skinned? Please clarify.