Whenever any part of the body becomes red, it can be a sign of a problem. Typically, the skin will become red as a sign of inflammation. This is one of the first ways that the body will react to foreign bodies. If you notice that your scalp has become red, there is a good chance that you have some type of scalp infection. Some types of scalp infections will clear up on their own in just a few days.
Other times, when the infection becomes more severe, a person might need to go to the doctor in order to get a proper diagnosis and treatment for the problem. If you have recently started using a new shampoo or other hair care product and notice that your scalp is red, you might simply be having an allergic reaction. If you stop using the new product for a few days and the redness clears up, you know that this was likely the cause.