The jobs cost is determined first by the attributes that the page counter has detected which are then applied to the job cost method to calculate the cost. View the spooled job in the queue using Pharos Admin/Remote to ascertain what the pagecounter is finding. I expect that the generic drivers grayscale is actually being detected as colour. Make sure you've got the latest page counter on your print servers and if its still detecting gray scale as colour, send the spool file to Pharos support to check for you, they may be able to advise a setting to change or get the pagecounter updated so that it detects what you want.
Its absolutely possible to do what you want on a single queue and we've done it for years. BUT the pagecounter has to correctly detect the attributes that are sent for it to work.
We have a print queue set up like that. From our experience, when they say B&W it really is just black and the absence of any color (white). If there is one pixel that is not black, it is charged as a color print job. It is possible that the conversion to greyscale is producing shades of grey instead of dithering (kind of like a newspaper photo) which is not black, so therefore it is a color. When students send their reports or papers which are just text to the color printer, they are charged a B&W cost.
Santa Clara University
I feel like I should mention our experience charging the same for color printing on paper of any size. When word got out that we were charging the same price for ledger (we call it tabloid) some student groups began mass-printing posters with Uniprint because we undercut the price of other local printing services. The problem was these posters were so saturated with toner they cost more to print than we were charging.
We doubled the price for tabloid color. The service is still just as popular but we're back to breaking even instead of losing money.