Dr Ph Martins makes a Skin Tone set
http://www.dixieart.com/Spectralite_...Ph_Martin.html set #3 on that page.
Thats one way to go. The other is problay the best and that is experimenting on your own with different kolors which is what I did with House of Kolor to come up with a set of skin tones I use all the time. It just takes mixing spraying, mixing spraying until you get a few basic skin tones. Light, medium, dark.
With createx give burnt sienna (a red brown), raw sienna (a yellow brown), and white. ( I don't use createx so I'm taking an educated guess as to their colors)
Hope that helps a little, hopefully someone else has a better answer.