Since there is no mask in the “Compositing Editor” of Blender yet, I found a simple tricks which could work pretty well especially in case of color grading.
You’ll see nothing really fancy here since the mask can only be square (or pretty close to a square shape though). But if you check out Colorista for instance, the two allowed shapes are ellipse and rectangle.
Blender Quick Tips : How to create masks in the compositing editor from François Tarlier on Vimeo.

















Great video and awesome colour balance node.
[...] (based on this tutorial I did on mask, but much better and flexible, just check it out) UPDATE : add Mike Pan’s vignette, which is [...]
my pleasure
you should check out my post about Color Grading with Blender 2.5, there is an close but better approach for doing mask in the vignetting node. You don’t have to create white and black picture any more. (I don’t know why I didn’t think of it earlier).
With the technic used there, the mask will always adapt itself with the size of the original input. Much better !
Thanks so much for this tip and the work with Matt in the colour tool. I really appreciate it. Text entry for colour wheels would be good too.
[...] This post was mentioned on Twitter by François Tarlier and François Tarlier, Julien. Julien said: RT @francoisgfx Blender Quick Tips : How to create masks in the compositing editor http://tinyurl.com/y9ll9y3 [...]