How To Color Spacific Char Charters In Rich Text Box?

Question

Hello im working on a program that displays a gui with all the keyboard keys and a rich text box in C# windoes forms,
the user is supposed to input a text to the rich text box and then press with the mouse (for exmple) the A button 
and all the a the a chars will be in green color,but i dont know how can i color those specific car charcters.

any help would be more than appreciated