Quote - The shine we try to mimic in 3D, which is essentially the oils, can be duplicated more effectively with the top coat. Strangely enough, I've found some great combonations with ethnic skin (some character's I've made promo images for are Cuban/Asian, etc.) and using a color for the top coat that one wouldn't think is even remotely like skin, gives great results. 

One fellow I have with caucasion skin uses a top coat in the area of 192 (light grey) in concert with some diffuse tweaks. Utilizing a self-made exr light file for IBL, I've gotten several renders where you'd swear its a photo. Same for an african american female, and an asian female. Lickably delicious ;-)  The asian has a top coat that looks like an orange crayon... 

 

Specular color = coat IOR, and thus both reflection intensity and strength of the fresnel effect. The second part is important, higher spec color not only makes highlights brighter, but they will strengthen dispropotionately on surfaces directly facing the camera. It is VERY important to remember that almost nothing you encounter is going to have an IOR much higher than ~1.5, which would map to a spec color of about 55 in Reality. Going much higher tends to make things bizziarely semi-mettalic.

 

"top coat" in Reality = absoprtion color + absroption depth, this is the amount of light lost in the coat. Stronger absorption will be darker, especially at grazing angles.

 

Glossyness = microscopic surface roughness of the coat. Rougher settings will blur reflections, resulting in larger and softer highlights.