I will put my penny into the discussion.
To isolate color on a separate layer:
1. Add Curves adjustment and put it into luminosity mode
2. Make 'master curve' horizontal. (see the pic below) Now you see only hue + saturation info, because you've wiped out luminosity
3. Merge visible and change the mode of the merged layer to color
4. Hide merged layer and go curves layer; change blend mode to color. Now you see black and white image
5. Again, merge visible
6. Now activate the layer from step 3.
Now you have it - color on one layer and luminosity on another
What I do not know is: how to make practical use of it:)