John, are you talking about the slightly brown patches on the glans/head of your penis?
Pigmentation change is very common across the human body and this can often be all it is.
The other situation that can arise is when there is the regular presence of moisture in the area, and this can create various dermatological reactions, again this can occur in various locations on the body, such as the middle of the chest, again where a regular occurrence of moisture is present.
You could see a Doctor or Dermatologist about this - they are likely to give you one of several potential cream or ointment treatments to see if it will clear away, obviously if it is a change in pigmentation nothing will change that …