Here we go, finally have a shot I can post from my cheap Ebay ND400 filter.
This shot was very underexposed. I allowed for about 9.5 stops of extra exposure (I use the PhotoBuddy app for the iPhone to work out exposures) but I would say this particular filter needs more like 11-12 stops extra. A Hoya ND400 filter is specified as a 9 stop filter. This shot has +4 stops of exposure in PP (Lightroom) and the colour cast is mostly corrected (but not totally) by adjusting the white balance. Here are the settings from lightroom and the shot. Its nothing special but it does display the problems with the colour, and obviously incorrect number of light stops filtered. As you can see I am at the limit of adjustment for white balance, but the photo still shows a slight purple cast. This means you will not get true colours from this filter.