I assume you did this in RGB space, so no pretty prisms? I couldn't find any explicit mention. From the report I guess you didn't implement refraction at all?
Spectral leads to great images, but the color space conversions are such a pain to get correct.
It’s a lot easier to learn from than the pbrt one in my opinion.
There are ways to avoid having to do single wavelength rays (hero wavelengths or basis functions) but it’s tricky.