It seems you have developed withPost inflammatory hyper pigmentation after lichen palnus.You do not worry about the color change. It is is the normal sequel dark shade because of the activity of melanocytes. You can apply Kojic acid
or glycolic acid
or azelaic acid cream over the affected area. Use a good sunscreen and protect form direct sun exposure. Do not try to scratch any of the pigmented area as it can lead to further darkness.