If you have SimPE there is a really easy way to change an Ingame sim's skintone. First open the skintone you want to use in SimPE and search for the Skintone XML-file. Click on Family and copy the value you see. I usually copy it to a text file just incase I loose it.
Then open the Neighborhood Browser, then Sim Browser and open the file of the sim you want to change the skintone of.
After you have opened it, then click on more and on open Sim DNA.
Change the existing values of Skintone and Skintone range (both dominant and recessive genes) into the value you copied from the new skintone package.
Then save and exit your done!