Thank you for your question.
Very often redness on the penis that is painless suggests infection by an agent called chlamydia. This is most likely a sexually transmitted infection and can be seen in many cases who may or may not have symptoms such as discharge or pain. They may have some irritation when passing urine or mild redness. Treatment is straightforward but you need testing for other related infections and your sexual partner(s) will also need treating.
Other reasons can be allergies or viral infections. A doctor specialising in Skin And DVD will help you best.
hope this helps