How Can Penis Glans Discoloration Be Treated?

i am 43 and since last 4 years it has been noticed that a black colour shade developing in my penis edge (its an uncut, when you remove skin) and the interest, pleasure which a man enjoy with errected penis get dramatically reduced. I red in the web that cholostrol can be a cause of it. Now since last 3 month i completely on control of my cholostrol level and all having a very healthy life style. still the cituation doesnt improve. Even i tried some medicines like Muslipower, but it also doesnt give any results in my case. Please guide me what to do for a healthy sexual life?