Nikon D6 Reference Manual page 288

2
Select a preset.
Hold the U button and rotate the sub-command dial until
the desired white balance preset (d-1 to d-6) is displayed.
3
Select direct measurement mode.
•
Release the U button briefly and then press the U
button until the D indicators in the viewfinder and top
control panel and the L icon in the rear control panel
start to flash.
236
Achieving Natural Colors with Different Light Sources (White Balance)