This is made in Gimp.
Go from
to this
Others made using the same techniques:
This is meant as a guide, so I am not going to give every single number, it depends on the image
Take your base and duplicate it, set the top layer to screen if it needs lightening, basically get your base ready.
Now, flatten your image. Duplicate your image. Work on the bottom image. Go to Filters-Colors-Channel Mixer. Select your settings according to how you want to enhance your image. I wanted mine with a little more red/pink to it.
These are my settings: remember, make sure you have "preserve luminocity" checked.
Red:
r=100
g=25
b=-48
I didn't mess with Green
Blue:
r=1.5
g=-7.4
b=120
This is how my channel mixer layer looks
Now, on your top layer, set it to overlay. Then, I go to Tools-color tools-hue/saturation. I up the saturation in cyan and blue, then click master layer and set it lower saturation, but not all the way black and white.
You don't have to pick cyan and blue, if you want to do reds, yellows or whatever color you want to bring out, up the saturation on it.
Now, for mine, I added a new blank layer, and set it under the overlay layer. I made it a very dark blue and set it to Subtract at about 30 opacity, just to give the skin more yellow.