You can always render out the layer with an alpha channel, import that back to your comp and use the alpha.
I am not sure if once you apply the rotobrush the track matte would work, but worth a try- let me know if it works.
I guess, but that would not give me the desired result. I want a mask of the area I defined with the roto brush. Meaning, if I were to key out an arm for example, the area of the arm is full white, the rest is full black. Applying the Colorama effect would take the luminance levels of the footage (the arm) into account, which is not what I want.
I guess, I am just used to being able to have access to masks/mattes from Photoshop. Whereas the AE roto brush seems to "hide" the mask/matte it creates.