It has always been a problem when using colour inks to produce mono prints without a cast. Don't forget there is no such thing as white ink and the white in mono prints relies on the shade of the paper. So if you want true white then make sure your paper is brilliant white. The grey tones to black are made up from the rest of the colours which will leave that dreaded magenta cast. This is the reason I have all my mono stuff printed from a company that uses an optical printer rather than inks. No chance of any unwanted colour cast that way. This is the company I use and they are brilliant:
http://www.onevisionimaging.com/