Adjustment brush for skin tone?

This is the way that I do it:

1. Mask the face. I usually use Radial Filter and Color Range Mask.

2. Set Saturation to -100. The masked area becomes grey-toned. i.e. colorless.

3. In Color Overlay set Hue, I know from experience that the skin tone hue I generally like is 24. The Color Overlay tool uses the H and S parameters of HSL color. If you only know the RGB of the skin tone, create and save in a pixel editor an RGB color swatch with that color (my Hue 24 is 200/170/150) and display it somewhere on your screen. Click in the C.O. tool and drag the probe to the swatch and release. H and S are set.

4. Adjust the S (Saturation) value. I usually like to start at 100 and then maybe reduce it later.

5. Go back to the saturation slider. If you move it to the right you are controlling the blend. At -100 only the C.O. is contributing color data and as you increase it more of the original color is added back.

6. Adjust Exposure to taste.

On the left the image at import, on the right after adjusting the face as above (not the hand).

This is not a job for Lightroom! although the histogram can show the RGB values under the cursor position.

The Adjustment Brush can apply a "Tint" to areas of a photo, but it is influenced strongly by the underlying tones (luminance & saturation) of the image where brushed. So you cannot brush an 'exact' RGB color.

The Brush can also modify color by painting White Balance changes.