1. Enter the color picker dialog (open Paint, double-click color at bottom)
   
2. Click "Define Custom Colors"
   
3. Select a color in the right half of the dialog
   
4. Watch the little black cross/cursor/whatever
   
5. Press Alt+O
   

After following these steps, you should see the cursor move a little, and the numbers in Hue/Saturation/Lightness may change. The RGB values, however, won't change. Wonder what this is for?

BTW, if you use a program like WinExp to force the dialog size bigger, you will see a button with an "o" as its caption, with that as its hotkey.

Can you please explain what use this is and for what operating system?

The RGB values, however, won't change. Wonder what this is for?

The numbers do change, unless you happen to have picked solid black or solid white, in which case the brightness/luminosity will make the RGB values 0,0,0 or 255,255,255 regardless of hue and saturation. That's how color pickers work.

As for alt-o, it's the hotkey for the Color|Solid box (swatch). Nothing hidden anywhere.

The RGB values, however, won't change. Wonder what this is for?

The numbers do change, unless you happen to have picked solid black or solid white, in which case the brightness/luminosity will make the RGB values 0,0,0 or 255,255,255 regardless of hue and saturation. That's how color pickers work.

As for alt-o, it's the hotkey for the Color|Solid box (swatch). Nothing hidden anywhere.

No, it's the hotkey for that hidden button mentioned in the original post. The o may be underlined in there, but if you hack the window larger, clicking that button will have the same effect as pressing Alt+O.

And that matters how? They just decided to use that to trigger the event instead, it works just the same (same purpose). Big deal.