I'd like to add something about color shifts from ND filters.

Taking a look to datasheets we realize that quality filters have a really flat spectrum, ND filters only changes colors in case of bad filters in their denser version, in special with some variable density filters based in cross polarization.

...but there is another effect, long exposures on their own may change colors, color film have different reciprocity failure for each layer, we all know the need of M filters for Velvia 50 long exposures... but color shifts may also happen with long exposures and any color film.

...this is well known, just pointing that (IMHO) there is a myth around saying that strong ND filters changes colors when it's about different reciprocity failure of color layers.

This can be seen in a single image: say we expose for 4 seconds (with ND filter ), while "Zone-V" is neutral it may happen that shadows (that would require 32s, for example) would have a color cast. This cast disapears if we can open up the diafragm (with ND filter still in place) by 3 stops.

For BW film it also may happen that spectral sensitivity changes with exposure, but I'm not sure about that...