Canon EOS 400D Digital Instruction Manual page 87

If the exposure obtained in step 1 is way off, a correct white balance
might not be obtained.
If the image was captured while the Picture Style was set to
[Monochrome] (p.64), it cannot be selected in step 3.
Instead of a white object, an 18% gray card (commercially available) can
produce a more accurate white balance.
Select [Custom WB].
2
Under the [m] tab, select [Custom
WB], then press <0>.
The SET screen will appear.
Import the white balance data.
3
Select the image that was captured in
step 1, then press <0>.
The data will be imported and the
message screen and menu will
reappear.
Select the custom white balance.
4
Press the <XB> button.
Press the <U> key to select <
then press the shutter button halfway.
B: Setting the White BalanceN
>,
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