Does Lab color mode in Photoshop give some advantages over Adobe RGB on skin retouching/frequency separation? If yes, in what cases/what advantages does it give? What would be the proper way of making use of these advantages?
Alright well AdobeRGB vs Lab in this case isn't really the benefit or concern so you're kind of asking the wrong question. There's really two questions:
Do the channels available in Lab make up for the greater number of tools available in RGB?
Is it better to do things like Dodging and Burning on an RGB composite or on a Luminosity Channel?
The first question will depend on your workflow, preferences and understanding of things. RGB tools have become in some ways more user friendly. In some ways because its also made people believe incorrectly that they no longer need to understand the underlying principles. What I mean is almost everything is really being done within Channels and Curves but
RGB has access to more filters and "one button tools" than Lab, among those are the following adjustment layers:
- Selective Color
- Frequency Separation
In a lot of tutorials you'll see people using Selective Color to correct skin tones. If that's what you're used to it might be scary to not have access to that tool.
That said, in Lab its actually even easier to correct. But you need to learn how or know enough about how everything works to know on your own.
It's also important to understand your image and the channels within it. When you need to make masks you may find one color space can get to that particular channel much easier. Likewise don't overlook Blend-If Options which is often much much easier within the Lab space, but again not always.
The second question is equally subjective. Are you doing micro dodging and burning to remove skin blemishes or large dodge and burn to contour and shape the face? In either case I find two methods to work fairly well:
Either I Dodge and Burn in RGB or I dodge and burn in Lab (or even Grayscale) and then apply it as a Luminosity Layer over top of the composite. In either case pushing Dodging and Burning to extremes will require additional tweaking and adjusting to get even contours and colors.
As for the Freq Separation question as I mentioned you can't do it within the Lab Colorspace.
Don't be scared to go back and forth between the two as needed relying on copies, smart objects, or even flattening if you have to.
I'd say if you're just starting out with either retouching or Lab then start with some basic Lab corrections then do your retouch in RGB then switch back to Lab at the end for a final look. Then as you build experience you'll start to know what you want to do in each.
I've started my own set of YouTube Tutorials that often use Lab if you'd like to take a look but one of the reasons I'd say to start in RGB if you're new is because other than my tutorials most are only going to show you how to do it in RGB. If you have any questions or requests though let me know and I'm happy to make more tutorials as time (and an aging computer) allows.