I start by getting my WB 'right'
Then I use a hue/sat adjustment layer like this:
- Select reds from the drop down.
- Max out the hue and sat sliders until the skin goes a really strange colour.
- Adjust the little sliders on the colour bars at the bottom so that only the reddest patches of skin look strange.
- Move the saturation back down to 0 or -2 ish
- Move the hue down to around +8
- Boost the lightness to +2
- Fiddle around with these numbers until the red areas more closely match the rest of the skin
Mask off where not required, e.g. lipstick, cheek blusher.
That'll get you a long way without too much pfaffing around and won't accidentally cause the sort of degradation that clumsy frequency separation can introduce.
Here's a video:
View: https://www.youtube.com/watch?v=ecMMp6Fc_Xg
Once the tones are close you can do whatever you want to alter the overall colour. Typically for Caucasian skin you'll want the green value halfway between the red and blue values (use the Info tool window)
To fine tune I sometimes use targeted curves adjustments, sometimes frequency separation or a variation thereof. Sometimes just painting on a layer set to Hue mode will do it. I rarely use selective colour adjustments. Sometimes I use blend_if. Occasionally I make a selection using channels. Sometimes dodging & burning will do it, but that can often need further colour correction.
If you want more details on the fine tuning just shout, I don't want to overwhelm you with technical stuff if it's not relevant.