I set up a target on a driveway. Is it a good target -- don't know, but it's a pretty plain scene, the entire view (and I was on matrix metering if it matters -- I still don't know for sure). It was bright sun in a cloudless blue sky, so the light was very constant.
I took four shots each, just to confuse you (but really because I didn't swap cameras first) the D4 is on top, the D800 on the bottom:
Auto-1 Auto-2 Direct Sun Force temp = 5000 K
The D800 looks warmer, and if you look at the histograms there is a pretty consistent red (and only red) shift:
At first I thought that one was more sensitive generally, and it was an exposure difference, but what I'm seeing is the D800 having more red, and less yellow from what is otherwise the same image (on auto, so it is perhaps shifting). These are from ViewNX by the way.
Adobe shows those two at (4400,-7 for D4) and (4750, +3 for D800).
Below is the histogram for the fixed "K" setting, where in theory the color rendering on conversion should be identical. They are closer, but they are not the same -- the red is shifted a bit higher on the D800. That's consistent throughout the shots.
It seems as though the sensor is actually more sensitive to red on the D800? Is that how to interpret this?
But perhaps NOT on the AF sensor, which sets the "auto" (and maybe a bit the reverse), so the result is auto is warmer?
And in warmer (lower temperature, oddly) light like incandescent it doesn't matter as much in some fashion?
I went back and looked at the indoor shots, though, and the red peaks are shifted right on the D800 there also, including on the fixed presents like incandescent.