Can I color the empty space between two objects? How do I color in the space between the torus and the cylinder? I want to do this without changing the color outside the torus.
you have many options :


create another material with a different color
select the face you want to color and click on assign in the Material tab




OR


create another object that fits inside the torus and give it another material




OR


use a texture  and unwrap the object to map it correctly