In older Onyx v7.0 versions there was a severe problem we found when generating new colour profiles that created a problem with GCR (Gray Component Replacement).
When selecting a lower ink restriction within Media Manager the gray and black sections of the images being produced came out very green. In later versions this became better and more neutral although not correct.
In testing the absolute latest revision of Onyx Postershop/Productionhouse as at this post date, I have found that the advanced Grayscale still demonstrates a similar GCR problem.
Our testing has been on a 12 colour Canon IPF8000 with RGB inks (please comment if you have found other printers to be ok).
I suggest only doing a simple linearization within media manager while they sort out the problems.