Surface state and colorimetry—is it possible to assay gold with a smartphone?

Identifying ternary gold alloys by their mere colors appears to be problematic. Surface state induces multiple reflections and creates mixtures of structural colors. Two models stochastic backward raytracing (SBR) and finite asymmetrical dihedral facets (FADF) are explored to compute the color of a given surface state. Assaying gold by its color without a touchstone seems hopeless but using colorimetric microscopy and variable magnification could be a way forward.