Dark purple colouring on baxk of my penis.

Patient: Ok, so im 13 and i have a question or kind of a problem, so, the base or ‘shaft’ of my penis is ok untill i get and erection and the back of my penis just under the head is dark purple for like a CM down i cant imagen there is anything wrong but i would just like to know, it doesnt cause any discomfort and, ive bin trying to wash regularly but I have a very sensative penis which is very offputting,Thankyou for your time, i hope you reply.

Doctor: