STEP 1:

Activate the layer of the person whose skintone you will be working on and decide whose skintone you will match it with. (I will be matching Peter with Claire).
First you go to "replace colour".
http://i.imgur.com/1l2sIAN.png

STEP 2:

You will get a pop-up like the one below with a colour selector as your mouse. Click on the skin of the person whose skin you're changing, starting with the main colour.
Use the 2 last 'switches' to adjust the tone of the skin to try to get a closer match the person you are comparing it to.
Repeat the same process with the different colours of their skin, you will most likely have to do this 2 or 3 times.
http://i.imgur.com/lSG9Uwp.png

STEP 3:

Your skintone will be matching very closely right now, but often because of lightening etc,
a character might have a certain glow in a certain colour shade. (Claire has a bit of a red/purple/rosy skin glow).
So you can now open "Curves" (IN THE SAME DROP DOWN MENU WHERE YOU FOUND "COLOUR REPLACEMENT, NOT ON THE BOTTOM RIGHT)
and go to the shade you believe it to be and adjust the curves accordingly.
http://i.imgur.com/Mz1lqML.png