Alright two primary ways how saturation shifts in skin. The first reason is material, the reason they use a saturated color for the "transition" is because of the subsurface scattering, but do not be fooled, that subsurface scattering effect is only really visible in the shadow areas, A LOT of people get this wrong, and think they can stretch that saturation gradient into the light side, but that is reason #2. The second reason why there is a saturation shift in the skin in the light side is much more the second reason than the first, but they do combine. Subsurface scattering will not apply to opaque objects, but you will still see a saturation shift when shading objects. The second form of saturation shift happens because of the saturation level of the local value and the light source. If you have a blue sphere that is moderately saturated being lit by a white light with a barely yellow tint, (give the saturation of the passive highlight a 5 on the photoshop scale) the light will gradually accelerate to the sphere's blue local color saturation (give this saturation a 40), the rate of the gradient is exactly the same as the rate of falloff, but make sure add the ambient light. You have to factor both in to get a grasp on how saturation works with skin. One last thing there is a little subsurface scattering to account for in the light side of skin but the light source will overpower most of it, it does add a tint though.
TLDR sss saturation mainly appears in shadows and lightly changes the skin's tint, saturation gradient in light area has to do with local saturation of skin shifting with saturation of light source. Also don't do what i did and make the rate of saturation change linearly when making the gradient, it accelerates like a curve.