If the tooth is changing after one to six months of trauma or fall it indicates that the nerve and blood vessel in and around the tooth is loosing its life slowly. It will be confirmed either with an x-ray or a sensation assessment test.
If confirmed that the tooth has lost its life the dentist will suggest you to have a root canal treatment of the tooth. Most of the time discoloured tooth will not provide pain and even if pain is manifested, it is of a very low grade discomfort. So low grade pain is a sign of a big hidden problem in your tooth. So take attention immediately, sometimes there is a mild pus ooze, gum boil, loosening (etc) around the tooth.