short thought of simple mind w/ eye on matter...
Dielectric first.
Its color (light wavelength) gets defined by absorption and scattering properties of medium (volume substance).
Sub-surface, transparency and/or translucency are then basically one and the same thing, just another medium property defined and differed by purity/clarity (homo-/hetero-geneity), density and thickness of its form.
Metals are much simpler and generally exhibit solely surface properties (exception being 100s nm thick - thin film).
Q's:
1. Dielectrics - How is nesting coming along?
2. Metals - Will spectral data get support (ie. sopra or n,k values)?
Some gentle mindset stirring is taking action.
:)