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