ND question

As many people ask, I am attempting to do a review of the Fader ND from eBay. Here's my question for discerning minds:

Assuming F-value and light conditions remain consistent, you take one snap with nothing (true SS, so to speak), one shot stoppoed down 2 light stops and one shot stoped down 4 light stops, should all three pictures look the exact same short of any difference caused by movement (be it water, car, person, so on)? Would you see slight differences in color?

Some ND filters block some wavelengths of light more than others and so can give a slight tint. A physically perfect ND filter would cause no change to the photo except for differences cause by shutter speed. I guess flaring could also be an issue, but unlike the tinting it's a situation specific problem.

You shouldn't see any colour shift if the filter's truly neutral. In testing, I'd use a custom WB set with no filter (preferred), or whatever preset is appropriate for your lighting.