How do I get a grayscale image in a CMYK document?

Grayscale images that contain CMYK colour will have a colour shift when printed. That shift may be green or yellow.
Always check the CMYK values of your grayscale in the final CMYK document. If there are values other than black in your grayscale image, there is a chance that the colour will vary.
To eliminate all values other than black, use your Channel Mixer (adjustment layer) in Photoshop, then click “Monochrome” and adjust accordingly.