Color2Gray for InDesign allows you to convert placed color photos to grayscale without modifying the original color image.
Sometimes you want a particular picture to be output as a grayscale image, yet the original image is in color.
You could open the image in Photoshop and convert it to grayscale, but you can instead also use this plug-in. When using this tool, the original image is not changed and remains in color, yet it is output as a grayscale image.
When outputting in CMYK, the gray image will only render on the black (K) plate.