On the right side you can see the settings for the colour picker. Here's a short description of everything there:
- Sample average - set whether to pick average value from pixels within some radius (1 to 300px) of the selected point.
- Sample merged - set whether to merge the colour from all visible layers and pick that (i.e. to pick the colour as seen).
- Pick mode - modes, see below.
- Use info window - set whether you want to see the info window, see below.
There are four modes in the colour picker tool. Here are their descriptions:
- Pick only - only picks a colour, doing nothing to it (information dialogue must be turned on);
- Set foreground colour - picked colour is used as foreground colour;
- Set background colour - picked colour is used as background colour;
- Add to palette - adds the colour to the palette, especially useful while creating palettes;
Ctrl switches between Set foreground colour and Set background colour, when either of these is active.
Shows an info window, with two columns of information - you can choose for both of those these options: pixel, RGB, HSV and CMYK, with pixel and RGB being active by default. Shift toggles it on and off.