This question made me curious, so I tried an experiment using one HD polarizer and 3 ND filters (HMC ND8, Pro 1D ND16 and HMC N4D00) and a white-balance and color-chart.

My first observation is that the camera produces highly varied results when left to its own, particularly with exposure.

Using Multi-Segment metering:

• The HD polarizer gives a +1 EV delta which is perfect.
• The ND8 gave a +2 EV delta which is one stop short.
• The ND16 also gave a +2 EV delta which is two stop short.
• The ND400 also gave a +2 EV delta which is way too short, which suggest I reached the metering limit of the camera.

Under the same conditions with Spot metering:

• The HD polarizer gives a +1 EV delta which is perfect.
• The ND8 gave a +2.5 EV delta which is half a stop short.
• The ND16 gave a +2.5 EV delta which is 1.5 stops short.
• The ND400 gave a +3.5 EV delta which is way too short again. This shows the limit of spot metering to be lower than multi-segment.

Manually setting the exposures to the correct amount according to the strength of the ND filter gives perfectly consistent results in terms of exposure but shows variations in white-balance.

Given a filterless exposure of F/4 1s ISO 200:

• The HD polarizer needed 2s and had virtually no effect on white-balance. I knew this actually since it was the reason I sold my B+W filters which gave terrible color-shifts and replaced them with Hoya HD Polarizers.
• The ND8 needed 8s for the same exposure. White-balance however became noticeably more green. The grey patch on the color chart with RGB values (199, 199, 199) unfiltered became (189, 202, 179).
• The ND16 needed 16s for the same exposure and had absolutely 0 effect on white-balance, keeping the grey patches all perfectly neutral.
• The ND400 was exposed for 401s which gave the same brightness but gave results a slight bluish tint. The RGB (199, 199, 199) patch became (199, 213, 216), so actually less red.

In observation is seems the ND filters are well calibrated in terms of exposure but the camera struggles with lower light levels. Other than the ND16 Pro 1D (and HD Polarizer), these ND filters seem to have an effect on color. Having no multiple copies of the same one, I cannot say if this is a sample variation or simply properties of the filter.

The only thing to do to get WB perfect is to shoot RAW since the reduced light-levels do not allow a Custom WB reading to be made with the filters on under moderate light conditions at least.