Yes, basically the Hero Wavelength algorithm picks and importance samples an entire collection of wavelengths (as many as you ask it to) per pass to figure out what color a pixel has. A naive algorithm only considers a single wavelength at a time and that causes a TON of color noise. I think if you use Hero Wavelengths with only a single wavelength sample, it reduces to that naive algorithm (as there is only a single light to consider and so importance sampling becomes a non-factor). The more wavelengths you add, the more it can nail the correct color in a single go. In practice, at 8 wavelengths, the discrepancy is really tiny. At I think it was 10 it basically is completely gone. But at 4 it still is quite noticeable.
That said, to be clear, this is just as all other noise. Even with the naive algorithm, eventually the correct color emerges. And with 4 wavelengths it can happen fairly quickly too. But clearly at 128 samples in the above scenes it’s still very noticeable
I mostly noticed on shiny surfaces when flicking back and forth. It might really just be a seed/RNG difference. But check out the sheen on the dark cupboard on the far left of the image in the foreground, as well as the caustic on on the bluish bottle on the far right sideof the image (top of the shelf) as well as the reflection on the arm of the light right next to it.
Looking at it again it’s also possible that it’s quite the opposite and those are things that would go away with more samples. At this point I’m not sure and it could go either way