I used the masking in the UV Image editor - change the View to Mask, and ctrl click to add a mask point, keep doing so to trace the area you want to mask, and select the end points and alt-c to close. Use a Mask node in the compositor, and use it as factor for the color mix node and use a blending mode that makes you happy. I use just the image input node through my chain to a viewer node, and use a uv image editor window set to viewer to get the output, F3 over that and save to the image name with an incremental number so you can make changes and save your progress. Just make sure to turn off ‘Use Nodes’ while painting, since it will keep compositing on every paint dab or stroke.
Also, I use two different masks, since Blender allows as many as you want