Colour Picker Tool

Sampled (or picked) colours from the Colour Picker Tool are stored in the swatch next to the tool's icon on the Colour or Swatches panel.

The following settings can be adjusted from the context toolbar:

Apply to Selection—when selected (default), the picked colour will automatically apply to the selected object's Fill or Stroke, depending on which colour selector is active on the Colour or Swatches panel. When this option is off, the picked colour is not automatically applied.
Source—determines from where the tool can pick colours—Current Object restricts picking to the currently selected object; Global picks from all objects, irrespective of layer and selection.
Radius—Choose to sample from directly under the cursor (Point 1x1) or from differently sized square areas where the colour is averaged.