Indeed, the node-based Compositor will be where you can achieve the final results that you want. Here, you can indeed eliminate the colors – or, completely replace them.
In this case, you can simply use the Alpha channel, which represents transparency. Everywhere the girl is … is “opaque” (Alpha=1.0). Everywhere the girl isn’t … is “transparent.” (Alpha=0.0) So, bingo … “there’s your outline.” You could simply grab that output, mix it with a color-node that directly produces the shade that you want, and use that as your output … ignoring completely the one produced by the render! (But, do pay attention to “anti-aliasing” along the way, to avoid “jaggies.” Batteries are included to handle this.)
Yes, the compositor is “the digitally perfect darkroom.” In which you are free to creatively concoct any sequence of processes that you can “plug up.” Make it your business to thoroughly learn about this tool and all that it can do for you.