D750 manual spells it out, pages 155-161. This WB setting is then only good for the same lighting that was used on the card.
Thanks for the page reference. I
was doing something wrong.
I was pressing the WB balance
and holding it thinking it should take the color reference photo. I was perpelxed because sometimes it would work and sometimes it would not. That was because I was pressing the WB button over and over again out of frustration until it worked.
Pg.157
"Release the WB button briefly and then press the button until "PrE" starts to flash in the control panel and view finder. The displays will flash for about six seconds."
"Before the indicators stop flashing, frame the reference object so that it fills the view finder and press the shutter release button all the way down..."
Simple fix: Press the WB button TWICE quickly, and HOLD it until the PRE button shows. Then press the shutter button completely. The camera will say "Good" when it has adjusted the WB properly. I can now adjust my WB in seconds and I have no more frustrations with the camera.
Fun Tip:
My iPhone6's case is the color "18% Grey".