Part I on how to fix some skintone density problems for Caucasian or dark skin models, can be found here

I am going to work on the same image as in Part I to demonstrate the different effects that can be achieved.

1. SKINTONE DENSITY: CHANNEL MIXING via APPLY IMAGE

Difficulty Level: Takes some concentration

When in the workflow: for uneven tone correction - right in the beginning of the retouching, before I do any blemish removal - as I need to blend the corrected skin area. For a tan effect - after I have finished with skin retouch.

1. Duplicate Background Layer

2. Name New Layer: Skin Darker

3. Click on Skin Darker Layer to select it - not just make layer visible

4. Change Skin Darker Layer to Luminosity Layer Blending Mode

4. Open Channel Palette

5. Click on the Red Channel 

6. With Red Channel selected, go to Image > Apply Image, enter the following settings:

Basically, we want to blend Green Channel, that has more detail, into Red Channel, which mostly accounts for the skin tones in Caucasian models (and in current image is not "dense" enough) 

Your finished image should look something like this:

NOTES:

1. Although Red Channel + Green Channel is my "to go to" starting point for a Caucasian beauty, I would often experiment with Red Channel + Blue Channel, often on lower opacities. The Blue Channel contains much more "skin grunge" and it will become quite visible, unless the file is properly cleaned up before the density adjustment.

2. Increasing skin tone density will bring skin blemishes, small scars & pigmentation problems to light - be ready.

3. For African skintones, which are not necessarily controlled by Red Channel in most cases, I experiment with Green Channel + Blue Channel on various opacities.

4. Adding Layer Mask for better control over other areas of the images is advisable.

Hope this makes sense, if not, please feel free to ask in the comments bellow - i will adjust the post to match your questions.