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