Pentax K-r Operating Manual page 58

You can assign one of the following functions to the _) button and access
the function by simply pressing the button while shooting.
Green Button Resets the values being adjusted. (default setting)
Custom Image Sets the Custom Image settings. (p.213)
Optical Preview Displays the Optical Preview. (p. 139)
Digital Preview Displays the Digital Preview. (p.140)
Digital Filter Sets the Digital Filter. (p.155)
Cross Processing Sets the Cross Processing. (p.216)
Temporarily changes the file format. Simultaneously saves
One Push File an image in both JPEG and RAW format, regardless of the
Format [File Format] setting. You can set whether to apply the
setting to only one image or not, and select the file format
when the _ button is pressed. (p.198)
1 Select [Green Button] in the [1_1 Rec. Mode 4] menu and
press the four-way controller (•).
The [Green Button] screen appears.
Green Button
Action in M Mode P LINE
E-dial in Program P SHIFT
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Press the four-way controller
(•).