Printing color as black and white (or greyscale)
 

I have an OmniGraffle Pro document that has a color PDF file at the base layer (a building floorplan), onto which I've superimposed new details in black and white.

When printing this document on laser printer, the lighter colors of the base PDF document print as very, very faint lines, making it very difficult to see.

Ideally, I'd like a 'print color as black and white' option for this document, much as Visio has.

Is there any such option in OmniGraffle? Or if not, any way to convert a color document (or color PDF) to black and white?