Please look though your question one extra time before posting, and make sure what you ask is clear and easy to understand.
Let's say you want to roto a person. You create a shape for the face, one shape for the hair, one for each arm, one for the torso and a couple of shapes for the feets.
If you only use one single MediaOut node (with index number 0), you will only get access to the full roto shape (including face, hair, torso, and feets) in the color tab.
So, what If you want to access to the hair only? Because you want to colour it blue? Then you need to connect the roto shape of the hair to a new MediaOut node (with index number 1)
If you then create an extra source / input in the color tab you will get access to the roto of the hair.