LAB COLOR

What's ironic, if that's the right term, is he is just making the Lab move to build a luminosity layer set to Screen (which makes the image in Lab look butt ugly). He could have duplicated the original image, converted to Lab, built the layer and shift dragged it onto the original RGB document. Is maybe one more step (well its about the same, you don't need to convert back to RGB). Its interesting to see the difference in color kick in when you convert from Lab back to RGB. If you try both techniques, the net results, especially based on what he's trying to do (paint in highlights) look about the same but with the duplicate technique, the original image doesn't have to be converted to Lab. All the stuff you paint into this image from the moved layer of course did undergo the Lab damage.

Then I found another potential issue. The original document I opened was in ColorMatch RGB, my working space is set for ProPhoto RGB. If you do the RGB to Lab and back, you end up (in this case) in ProPhoto RGB, not the original color space (ColorMatch RGB). That's not an issue when you just drag the Lab created layer back to the original RGB document.