Thank you for your question! It is very normal for a person’s penis to have some color changes, such as lines of purplish-red, during times of arousal or anytime when the penis becomes erect or ‘hard’ (such as with wet dreams, masturbation, or sex). When the penis is ‘hard,’ there is increased blood flow to the blood vessels and glands of the penis, which can make the skin surface look purplish-red. The change in color usually goes away when the penis becomes flaccid (soft).
However, in some rare cases, a change in color of the penis can be worrisome. If you have other symptoms such as: severe pain, itchiness, a sore on the skin of your penis, or burning when you pass urine (pee), especially if you’ve had unprotected sex in the past, it could mean that you have an infection or there may be something wrong with your body’s circulation (or blood flow). The only way to know for sure is to get checked by your health care provider.