Completely new world for me so be gentle.

This is with tracing four wavelengths at once; three of them are killed when a wavelength dependent pure specular dielectric is encountered.
The tracer accumulates CIE XYZ colors and converts them to RGB using the approach proposed by Brian Smits (http://www.cs.utah.edu/~bes/papers/color/paper.pdf). I chose not to use the more accurate method of Meng et al., (https://jo.dreggn.org/home/2015_spectrum.pdf) as it requires substantially more involved calculations.
EDIT: another one:
This is unidirectional path tracing so the caustics took a while to converge, and they are still noisy...