Being angry is a condition which cannot be described as permissible or forbidden. Prohibition applies to actions which a person may do of his own free will.
Anger is a reaction to some sort of an event which is particularly displeasing to a person.
However, anger tends to cloud one's vision. It causes a person to do things that he may not choose to do in normal circumstances. Therefore, one should try as much as possible to cool one's temper, and not allow his anger to get the better of him. The Prophet has spoken about the need to control anger. It is mostly through common sense that a person can resist acting in anger.