If you submit a file saved in greyscale, you can expect a color shift to occur when we convert your file to CMYK that will apprear as a green or yellow tint to your greyscale within the final print.
If you are wanting to obtain a "greyscale" image using your CMYK document,
Check the CMYK values of the grayscale in your final document. If there are any other values other than K in your grayscale image, there is a chance that the color can vary.
To eliminate all values other than K, use your Channel Mixer (adjustment layer) in Photoshop, then click "Monochrome" and adjust accordingly.