Canon EOS-1D X Mark III Advanced User's Manual page 134

6.
Select [OK].
The custom white balance will be registered to the camera.
Check the message and select [OK] to return to the screen in step 3.
Caution
These images cannot be selected in step 5: Images captured with the Picture Style
set to [Monochrome], multiple-exposure images, HDR images, Frame Grab
images from 4K movies, and images shot with another camera.
Note
To shoot with a registered custom white balance, first select the number of the
registered custom white balance.
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