What Causes Pink Colouring On Penis?

I'm wondering if my husband has been having sex with someone else based on the look of his penis. he is a black man whom was circumcised at birth. There are times when his penis is a bright pink from the head about half way down the shaft then it is black from there to the base. it ;looks like he has has sex but couldn't get his penis all the way in. Do you have any answers.