How do I get a neutral gray image in a CMYK file?
File Setup & Color
When a grayscale image is converted to CMYK, it usually picks up some cyan, magenta and yellow along with black. On paper that can show up as a green or yellow tint in what should be a neutral gray.
Check the color values of your gray image in the final CMYK file. If anything other than K (black) shows up, the color may shift.
In Photoshop, add a Channel Mixer adjustment layer, check Monochrome and adjust until the image uses black only. That gives you a clean gray on press.