The field tests showed no significant difference in color rendition and both cameras did a good job with setting white balance. We therefore decided to shoot our color checker to get a closer look at the differences, if any.
The shooting parameters were:
White balance: Auto
Indoors EV +0.5, outdoors no EV compensation
RAW, converted in Adobe Camera RAW with no color correction but adjusted to same exposure level
For reference we include the entire image from the outdoor scene.
Indoors, the Pentax 645D has an almost-perfect white balance whereas the Nikon D800E is a bit off. You probably will not see a difference in your web browser, but we checked the RGB values of the topmost row and found the Pentax to be spot on and the Nikon to have deviations from true grey. Both cameras reproduced the full tonal range.
Outdoors, both cameras showed minor deviations from the ideal.
But the bottom line is that for all practical purposes the two cameras perform identically.