gray printing
If you are tweeking the RIP when you print then all the other colors along with it will be thrown off and not matching PMS as they should be, the print K only in grayscale that was metioned is great but only if there are no other colors involved. It needs to be with profiles or at the source of your software (Photoshop, Illustrator, etc.) The dark grays are a problem, inside they look gray and outside in the sun either green or red grays or just the reverse with gray in the sun and inside with the color change. Try the different PMS colors such as black 7C at 90% or spot color CMYK at 14 0 14 75. As the other post mentioned the overlam will change the outcome also.