I had a similar issue with color modeling on our 700.. some pages would look fine.. others would look terrible.
I was absolutely positive that it was an issue with the machine... but one of our xerox techs thought a software update on the actual printer would work. So, he did his update and the color modeling issues disappeared (I can say that immediately after the update, the colors were awful.. it wasn't until I updated the software on the RIP [to match the update on the 700] that everything worked correctly.)
Good luck convincing them of any color issues.. Sadly enough, their idea of "off colors" seems to be quite different from everyone else. :/
It seems like Xerox will feed their customers whatever load of crap necessary in order to sell these machines.