Changing colours of the penis

It is normal for the head of the penis (glans) to change colour. Often it is quite red when aroused, and a more purple colour at other times. This is because the skin of the glans has lots of tiny blood vessels near its surface, so the colour just represents the changes in blood flow. If the skin is sore and itchy as well as red, or looks abnormal in any other way, you might have balanitis, for which you should see your doctor.