The Eye Dropper allows you to easily pick colors from the video stream. As long as you have either the Draw Tool or Disappearing Ink enabled, it will show in the annotations toolbar.

To use, simply select the Eye Dropper from the annotations toolbar.

Hover over the part of the video that you want to copy the colour from and click it with your mouse. The selected colour will now change and it will automatically revert to the tool you were using last.