Rich black is an ink mixture of solid black, 100% K, with additional CMY ink
values. This results in a darker tone than black ink alone. If you print black
alone as 100% K, the resulting black may not be as dark as you might like.
We recommend using C 60 M 40 Y 40 K 100 This will give you a deep, dark, rich black.
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.