Pentax Optio M20 Operating Manual page 90

If you use the [White Balance] function frequently, you can save time
by assigning it to the Green button. (p.56)
Saving the White Balance 1p.96
Manual Setting
Have a blank sheet of white paper or similar material for White balance
adjustment ready beforehand.
1
In the White balance screen, use the four-way controller
(23) to select K (Manual).
2
Point the camera at the blank
4
sheet of white paper or similar
material for White balance
adjustment so that it fits the frame
at the center of the LCD monitor.
3
Press the shutter release button
fully.
The white balance is automatically adjusted.
4
Press the 4 button.
The setting is saved and the screen returns to the [A Rec. Mode]
menu.
5
Press the 3 button.
The camera returns to capture status.
88
White Balance
Adjust
SHUTTER
Cancel
MENU
OK
OK