The eyedropper (pipette icon) allows you to sample from anywhere in the Blender window. The eyedropper can be used to select different kinds of data:
This is the most common usage, the eyedropper is used to sample a pixels color from anywhere within Blender.
The View Transform of the color management affects the color. In order to get consistent results, it should be set to Standard. If it’s set to any other option, the eyedropper may return an inaccurate color.
- Color Ramp
Dragging the cursor over the window to sample a line which is converted into a color ramp.
This is used with object buttons (such as parent, constraints or modifiers) to select an object from the 3D Viewport or Outliner.
- Camera Depth
Number fields effecting distance can also use the eyedropper.
This is used to set the camera’s depth of field so the depth chosen is in focus.
E will activate the eyedropper while hovering over a button.
LMB dragging will mix the colors you drag over, which can help when sampling noisy imagery.
Spacebar resets and starts mixing the colors again.