Hello.
What you have described here is normal. The skin of the genital area can have more of the Melanin pigment, which darkens the skin color. There are physiological reasons behind this. Also, having two shades of color on the penis is normal. Avoid getting too anxious due to this issue.
Hope this helps.
Abhijeet Deshmukh, MD
(This post is for the purpose of providing medical information and is not a substitute for a face-to-face consultation with a doctor. This post is not intended to give or rule out a diagnosis, create a doctor-patient relationship or replace an existing one. I am not able to diagnose medical conditions online. Please consult with your doctor or a qualified healthcare provider for diagnosis and treatment options)