When I applied m skin become dull or looked darker
Vitamin C serums usually brighten skin, but sometimes they can have the opposite effect. This might be due to oxidation (check if the serum has turned dark), sensitivity, or lack of sun protection (vitamin C can increase sun sensitivity). Try stopping use and consult a dermatologist to determine the cause and find the best solution for your skin. Don't forget to use broad-spectrum sunscreen daily if you continue using vitamin C products.