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You might want to invest in an IR-cut ND filter Boris.
Even the D800 has issues with strong ND's.
I've done a lot of long exposure strong ND work with my D800 and never noticed anything. To me, that example looks like the viewfinder being left open.
Bingo, you nailed it imho.
The D800 and 5D3 do NOT have issues with color casts due to long exposure of that sort. The D800 DOES have some amp glow at high ISO long exposure (but it looks NOTHING like that). In my experience, the D800 is slightly nosier than the 5d3 at ultra long exposures (though I usually use dark frame subtraction so meh w/e at that point). By slightly nosier I mean more hot pixels not the overall image.
An example of a D800 shot of approx 2 mins exposure, using an ND500 filter. It was under exposed. Whoops. User error on my part. I then boosted the entire picture up by over THREE stops. No color cast.
You will get color casts when using an ND400, ND100, ND500, ND1000, whatever filter in two cases:
1. You leave the viewfinder open and light goes in. You might think "that doesn't happen". And normally it doesn't. I never bother to close the viewfinder up with anything less than a minute long exposure, at least. You really hafta do something incredibly stupid to muck up the exposure. (Ie, you need to be like me and do things like under exposure your entire image by 3 stops!) But you know what...people just don't think about this. They'll go up into the mountains for a 10 minute star exposure. They'll focus, get it all right, then lower the ISO and start their 10min exposure. Then they'll stand behind the camera playing on their cell phone with the brightness cranked way the hell up. Then stuff like that happens and people wonder why. Ask me how I know! lol :P
2. Light rays going through the ND filter at an angle will be darkened more than light rays going through the ND filter straight through. This should be intuitively obvious to anyone with a little common sense. I would seriously hope. lol. As there are a total of 0 ND filters on the market that are 100.00% percent neutral...and the darker you go, the harder and harder it is to maintain a completely neutral darkening of the image... Well, some images, especially those taken with a wide angle lens (see above, this should be intuitively obvious again), will have a color cast in part of the pic. This can be corrected very easily with local white balance (or, I'd assume, even easier with capture one but I'm not a C1 user).