Nikon D300 User Manual (en)

If lighting is too dark or too bright, the 
camera may be unable to measure white 
balance. A flashing 
b a will appear in 
the control panel and viewfinder (at 
default settings, the displays will flash for 
about six seconds). Press the shutter-
release button halfway to return to 
Step 4 and measure white balance again.
Select preset d-0.
If the new value for preset white balance will be used 
immediately, select preset d-0 by pressing the WB button and 
rotating the sub-command dial until d-0 is displayed in the 
control panel.
Direct Measurement Mode
If no operations are performed while the displays are flashing, direct 
measurement mode will end in the time selected for Custom Setting c2 
([Auto meter-off delay], pg. 279). The default setting is six seconds.
Preset d-0
The new value for white balance will be stored in 
preset d-0, automatically replacing the previous 
value for this preset (no confirmation dialog will 
be displayed). A thumbnail will be displayed in 
the preset white balance list.
To use the new value for white balance, select preset d-0 (if no value has 
been measured for white balance before d-0 is selected, white balance 
will be set to a color temperature of 5,200 K, the same as [Direct sunlight]). 
The new white balance value will remain in preset d-0 until white balance 
is measured again. By copying preset d-0 to one of the other presets 
before measuring a new value for white balance, up to five white balance 
values can be stored (pg. 139).
Control panel