I just found a great way to combat rainbowing

[Katie Boundary]

Ttempsmooth seems to perform luma smoothing and chroma smoothing independently. Using ttempsmooth with a modest Lthresh value but an absurdly high Cthresh value, like 10 and 100, respectively, seems to crush rainbowing pretty thoroughly (at least in stationary parts of the image) without causing noticeable artifacts.

I do not recommend attempting this method with the ttempsmoothf version of the filter.

[Selur]

side note: basically any denoiser that also handles chroma can be used to combat rainbowing if you restrict it to chroma and just boost its strength enough,...

[Katie Boundary]

side note: basically any denoiser that also handles chroma can be used to combat rainbowing if you restrict it to chroma and just boost its strength enough,...

For temporal denoisers that have the ability to be restricted to chroma, sure, though I can't guarantee how many will yield an acceptable level of artifacts.