The easiest way, of course, is to print on a monochrome printer. There are
(at least) two other options:
1. You can change the picture itself. On the Picture toolbar is a Color
button that opens a menu that includes both Black & White and Grayscale;
choose whichever one you mean. This affects only the given graphic, not any
others in the document.
2. Your printer Properties may have a setting to print in grayscale or B&W.
This will affect all graphics in the document.
I have always been intrigued by the Compatibility Option to "Print colors as
black on noncolor printers," wondering if Word can actually tell the
difference. I just tried printing a color graphic to a color printer using
that setting (to see whether it might be a third option), and indeed it
printed in color. Since colors generally do print as grayscale on monochrome
printers, I'm not sure what this option is meant to accomplish.
