How to factory reset the D810:
Turn the camera on.
On the top of the camera, there are two buttons marked with a green dot. QUAL on the left. exposure compensation (+/-) on the right.
Look at the camera's top LCD. Press and hold both buttons marked with the green dots, until the LCD display goes blank. Release the buttons.
When the LCD comes back, the camera has been reset.
After doing the factory reset, the first thing you want to learn about the D810 is how to use the white balance controls.
Press the WB button in the four-button round assembly on the left side of the camera, and roll the thumb roller on the rear right of the camera. You will see in the LCD display that this causes the bottom row of icons to change. Start with A, for Auto. Refer to your manual for instructions on each white balance setting. There are icons that symbolize incandescent bulb lighting, fluorescent tube lighting, daylight (sun icon), and others including the K (Kelvin) settings. Start with AUTO mode first, then try the different individual settings in their matching light conditions and also in mismatched conditions, to see how they affect color balance.
If you're still not getting good photos, it may be time to at least get the camera thoroughly cleaned. I know, you said it's a new camera, but is it new, or just new to you? Maybe there's a cake of dust on the image sensor or something? Mice built a nest in your lens? Who knows? But try the white balance adjustments first. You can get some really OFF looking pictures if white balance is set wrong.
If you've done the reset sequence Don suggested, as long as WB is set right, you should get good pictures, no questions asked, if you have your exposure settings anywhere near right. I suggest you switch it to Program mode (Hold down the MODE button, rotate the roller until P shows up in the LCD display) and take a few photos in that mode. Between Program mode and Auto white balance, that's as fully automatic and foolproof as the D810 gets. If you can't get good photos then, you need to take it back to where you got it from for a hands-on diagnostic.
