Mockingjay is not just the bird it is the symbol of freedom and liberty in the Hunger Game series. It defies Capitol and its power. Wearing a Mockingjay Pin is the sign of rebellion against the Capitol Government.
The mockingjay with dark orange and red backgrounds indicates that the nation is boiling. A fire of rebellion is catching fast in the hearts of the people which is all ready to burn the Capitol at the slightest pretext.