The eyedropper lets you color-pick from your drawing rather than having to find the exact color in your color wheel.

The default shortcut for switching to this tool is I.

Pushing and holding Alt also activates the eyedropper for quick selecting of a color. It switches back to your painting tool when you release it.

In the settings panel on the right, you can switch where your eyedropper selects from. You can choose to select from the entire drawing or only from the active layer.