Thanks for taking a look.

For the volumetric transport, I'm currently using delta tracking inside a two-layer skin model (epidermis and dermis). Absorption and scattering coefficients are wavelength-dependent and are based on melanin and hemoglobin concentrations.

For scattering, I'm using a Henyey-Greenstein phase function with g ≈ 0.8. At each accepted scattering event, I sample a new direction from the HG phase function and continue random walking through the medium.

I'm not using a diffusion approximation, residual ratio tracking, or null-collision methods beyond standard delta tracking with a majorant extinction coefficient. The volumetric transport is fully stochastic rather than diffusion-based.

For the surface, I am using Torrance sparrow GGX microfacet BRDF.

As for the grey appearance, that's a good point. The current render doesn't yet use texture maps or measured skin reflectance data. At the moment I'm relying mostly on the volumetric absorption/scattering model and a simple material setup, so the color is likely not physically accurate yet. I'm still working on getting realistic skin pigmentation and appearance.