The following are copy from several different user's post.
Hopefully this can help more newcomers (or not) to know more about the great Green Button.
Thanks to Peter Zack, RuiC, sezme.

In manual mode (A,FA, DA etc series lens) you can push the button and it sets the exposure for the program mode and the program line. Program line is the setting you have selected in the camera menus, like MTF (where the camera sets the lens at the best aperture for that lens, which is programmed into the lens by the manufacturer). Then you can fine tune the exposure to your liking from this setting. Very old school but an absolute requirement for a serious shooter in tough lighting and many other situations. (Peter Zack)

Step Down Metering
With an M or M42 lens, you can use the button to stop the lens down and take a meter reading. For those that own these lenses, it's an approximate reading and you have to understand (with practice) the way your particular lens will meter on the attached body. In most cases, these lenses meter well wide open but tend to over expose in middle apertures. So you use the green setting recommendations as a starting place to adjust to the correct exposure. (Peter Zack)

Return to HyperProgram(P) after changes
In HyperProgram (P) mode you can turn the front e-dial to adjust exposure time other than P suggests and it changes to Tv mode. If you turn the rear e-dial it becomes Av mode. Then press Green-button it returns to P mode again. (RuiC)
image from Digiichi - Next generation hyperprogram.

From Full Manual mode to P, Av or Tv
The green button in M mode get you to P, Av or Tv dependant on what mode is chosen in the green button menu set up. (excellent if you want to use it like an on-demand spot-meter)

The green button doesn't change from M (manual) mode to any other mode - it leaves you in M mode. It just momentarily gives you the effect of being in one of those other modes (which one depends on how you've set it up in your custom options) - setting the same aperture and/or shutter speed that you'd get it you were in one of those other modes, but then leaving that set. As opposed to constantly readjusting as the light changes.

(thanks lol101, Marc Sabatella, adwb)

Act as Reset button: Reset Exposure Compensation, Flash Compensation
If you have set exposure compensation, hold the exposure compensation button plus the green button will reset the meter to zero E.C.
* You can also reset the Flash Compensation with green button. (thanks rockmaster1964)

Fast access to set Auto ISO
if you press the ISO button (OK button on K10/20) plus the green button, it sets ISO to auto. A very useful button!
Also in Av/Tv if ISO is made accessible via one of the control dial, green button switches to auto-ISO. (thanks lol101)

Zoom in in Playback mode
In playback, after you zoom, Green button zooms IN and EV+- zooms OUT

[Canada_Rockies]

The book I found on amazon was for K20D, again I don't know if it would be any help unerstanding my K-X

John

The controls are not the same on the two cameras, so the book will be of less help than the manual that came with your camera. Start with the green modes on your K-X and also the scene modes, but with the latter, look to see what the camera is doing when you change modes. You may gradually increase your attempts at control over the camera as you learn what it does. The Pentax manuals are better than some. I bought the Magic Lantern for my K10D and did not really get a lot of information out of it, but I have been using cameras for a long time.

[rubin]

Thanks for this post. This is Excellent at all!

[johnb-123]

Thanks for the information I will be using a screw mount on my K-X as soon as I get my ordered adapter.

[johnb-123]

I wonder why there is so little information on Pentax equipment? Thanks for your information on the books I mentioned.

[Canada_Rockies]

I wonder why there is so little information on Pentax equipment? Thanks for your information on the books I mentioned.

In my opinion, there are two reasons for the scarcity of books specifically on Pentax:

1. Canon and Nikon have a huge number of users, Pentax does not
2. Pentaxes tend to be much easier to figure out than the others

[boldy80]

I dropped my camera and the green button fell out. Luckily it was a K200D and there are no other scratches on the body whatsoever.

I have the button and keep it in place using sticky tape, it still functions as it should.

Due to the age of the camera, it is no longer under warranty and CR Kennedy replied saying they'll only repair it under warranty, if I post it to Melbourne from Sydney.

Can anyone recommend a repair shop in Sydney Australia? Or if I should tackle sticking this green button back in myself - how hard could it be?

Cheers,
Matt

[stephenh]

Didn't know this either, thanks!

[Postumus]

Sorry to hear you've lost you green button boldy80.
Kennedy's is expensive. Is the colour improtant? - Fire engine red never goes out of fashion. Green is for GO usually so I think of it as the GO Button!
All colours are equal! ................ Let's have some more red ones!

[Pepe Guitarra]

Can any one tell me what is CA? Not California, but the stuff that happens in you photograph...! Thanks,

[adwb]

Chromatic aberration - Wikipedia, the free encyclopedia

by the way this question is posted in the wrong thread

[Pepe Guitarra]

Sorry

[Nowhere Matt]

On the K-7, in Av Mode push the Green Button and use the front e-dial to adjust exposure compensation.

Edit - Upon further consideration this is only a matter of options selected in the Custom menu and pushing the Green Button is not necessary.
Silly me.

[Rebecca James]

Thanks! I didn't know about auto ISO.

[analyze7]

Very helpful thread. Thank you very much!!!

[slr_neophyte]

Fast access to set Auto ISO
if you press the ISO button (OK button on K10/20) plus the green button, it sets ISO to auto. A very useful button!
Also in Av/Tv if ISO is made accessible via one of the control dial, green button switches to auto-ISO. (thanks lol101)

I've been playing around on my K-x and haven't been able to notice the green button switching my ISO mode to AUTO. On the K-x the OK button is not the ISO button. I have tried pressing the ISO button with the green button, but nothing happens. I have tried this in M, Av, and TV modes - all to no avail.