Canon EOS 5D Mark III Instruction Manual (en)

You can correct the white balance that has been set. This adjustment 
will have the same effect as using a commercially-available color 
temperature conversion filter or color compensating filter. Each color 
can be corrected to one of nine levels.
This function is for advanced users who are familiar with using color 
temperature conversion or color compensating filters.
Select [WB Shift/Bkt.].
Under the [z2] tab, select [WB 
], then press <
Set the white balance correction.
Use <
9> to move the “ ” mark to the 
desired position.
B is for blue, A is amber, M is 
magenta, and G is green. The color in 
the respective direction will be 
On the upper right, “Shift” indicates 
the direction and correction amount.
Pressing the <L> button will cancel 
all the [WB Shift/Bkt.] settings.
Press <
0> to exit the setting and 
return to the menu.
 White Balance CorrectionN
White Balance Correction
Sample setting: A2, G1
During the white balance correction, <u> will be displayed on the LCD panel.
h> can be displayed in the viewfinder when white balance correction 
is set (p.324). 
One level of the blue/amber correction is equivalent to approx. 5 mireds 
of a color temperature conversion filter. (Mired: Measuring unit indicating 
the density of a color temperature conversion filter.)