I meant to add that if you have vitiligo, it isn't serious. It's a cosmetic problem only, with no other significant health implications.
Welcome to the STD forum.
No STD causes such changes in appearance to the skin of the penis, and STDs are the only topic for this forum; we don't get into non-STD problems that happen to involve the genital area. However, from your description, you might have a condition called vitiligo, a patchy loss of normal skin pigment that turns the skin lighter color. But that's only a guess. You need to see a health professional for proper evaluation -- either your primary care provider or a dermatologist. In the meantime, don't worry about STD. It's not an issue here.
Best wishes-- HHH, MD