just a few things, make sure your printheads are good, run a calibration, and make sure the original color has the correct profile - greys are very difficult, but you should be able to get a mix that has the correct color and prints clean.
DO NOT TRUST CUSTOMER ARTWORK for printable colors - there are a lot of colors that a customer likes in the additive display process of a monitor, but cannot be properly reproduced in the subtractive environment of inks on substrate. (reflex blue, the blue that used to be in pens, it cannot be reproduced through process colors, only through a custom mix spot. Your customer can very easily select colors that cannot be reproduced - and these will often start to show in the greys.)