The skin on the head of your penis is thinner than the shaft skin, so it changes color easily in response to blood flow. If you're Caucasian, it's normal to sport a red knob when you're aroused and a purple one when you're not. However, bright red – especially if it's accompanied by itching or pain – is a symptom of infection or an allergic reaction (possibly to latex). Black means your blood flow has been cut off; go to a hospital.