Any mask can be limited by luminance or color. If you want a basic luminance mask, do this:
1. Make a new brush mask
2. Click Fill
3. Go to the options area (looks like overlapping squares).
4. Move the luminance sliders.
There is a bug right now where the mask does not live update as you move the sliders. That will be fixed in the next release.
At some point, I may just add a mask that does that in one step, but that’s how you do it now.