Il find the Infinite realities file, and give you that later on, your see how iv set that up. The different colour maps depending on what SSS method you use willl have different outcome according to the overall skin tone. Referring to the Fast SSS2 which is a simple set up, the overal colour bears the most importance, the other two layers dont override this overall colour which is your standard pink skin colour map. The other two layers are for the colour of the SSS effect and hardly change the colour tone, where as the Skin material is totaly different. The Skin materials layers are all fighting againts each other and you have to fine tune the amount, and radius/depth of each layer, and the only way to do that is with alpha control maps. The case of the Skin material it takes ages to get something half decent, where as the Fast SSS2 not even close to the same time.
The diffuse colour stops light penetrating so yes it holds back the SSS effect which can be nice to give a bit of contrast back. To note if your model is not real world scale, you should use the scale parameter, not try to adjust all the other settings as your have a battle on your hands.
Skin as you know is not easy to set up as there are too many factors to take into account, its a ballancing act, but that can be easier if you eliminate some things right at the beginning which is proper lighting, proper scale, high quality model, and very high quality maps. Use the Vray Powershader or advbitmap loader to your texture maps into, this let you tweak the values. If Vray had a reliable IPR for skin, then it would make the world of difference getting instant feedback.
Dan