Colon is one of the punctuations which is used to: 
 1. Lay emphasis 
 Example: He loved just one: Bruno, his dog 
 2. To present dialogue ( we write the name of the speaker and then insert colon followed by his/her dialogue.) 
 Example: Rita: How are you? 
 Mary: Fine, and you? 
 3. Clarify composition titles ( to explain parallel ideas colon is used in compositions like lectures or book titles), and 
Example: Rousseau’s Dog: Two Great Thinkers at War in the Age of Enlightenment 
 4. To introduce text or lists. 
 Example: Three types of Subordinate or Dependent Clauses are: Adjective clause, Adverb clause and Noun clause