The Black and White Only preference is one of the quick preferences that you can activate on the toolbar. It is used to turn off all the color in the view, leaving you with just black and white objects. It will remove any greyscale colors.
This image shows the plan without the Black and White Only preference activated. You will notice several blue objects.
This image shows the plan with the Black and White Only preference activated. Notice how the blue objects have become black.
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