I'm sure someone more awesome will come along with a better explanation/method than this but all I do if I want to alter skintone is open my Sim's DNA (under other) and replace the code number for the skin they have with the number for the skin I want them to have. You can do this for both the dominant and recessive skin tones.
I notice that when I go into Sims2DB, it lists one short code for the skin, whereas in SimPE I get a long chain of stuff (something like "4dcb6e-000202-00005-3df6g9"), where the first block of digits matches what Sims2DB shows. Are the following chunks of data significant? (Because they vary from sim to sim, even between those that I thought had the same skin.)
To find out the skin number of the custom skin you want I think you do need a sim with that skin in the game. I guess one way to do this without putting unwanted sims in your game would be to create a custom neighbourhood and put in a CAS family with the skin/s you want. Then open simpe and find out the skintone numbers under Sim DNA - write them down (or copy/paste them into notepad) then use the numbers for the sim you want to alter. You could then delete the whole neighbourhood if you wanted.
That makes sense...but how about this case?
I've got a couple dozen custom skintones in my Downloads folder. I know that *one* of them is being used by the members of a family I am playing...but I'm not sure which one. Is there a better way of figuring out which skin it is, without the trial-and-error of creating a new sim with each of the possible skins until I find the right code pop up in the SimPE description?