If the printer is setup right and only tracking color the driver, if installed correctly, will use BI-di and automagically know to only track for color, but of course, the driver by default is set to print in color so it will prompt every time until you default the driver to BW, then it will prompt when you enable color in the job.
So first, ensure the printer is done correctly via
CWIS > Properties > Login / Permissions / Accounting > Accounting Methods > Service Tracking/Edit and make sure Color tracking Only is selected
Now, assuming the driver is setup correctly it will have this set already, if it is not, then you can choose the option manually, and when you do so it will give a message about the defaulting to BW thing. Start > Devices and Printers Right-click the printer and go to Printer Properties > Configuration
And to do the BW thing, you would need to do so in the printing preferences via
Start > Devices and Printers Right-click the printer and go to Printing Preferences > Image Options and set it to BW
This answer is the one for now, but it would not have been the same when this question was originally asked as the driver layout and the bi-directional communication has changed vastly on the device and driver in the 2 years between the top level post and your question now.
If you have not already resolved this issue, we suggest you search our online knowledgebase for this product: WorkCentre 7855.
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Has anyone had the issue when accounting is turned on for copy only that the print driver still requires a code even if it is only set to account for example color copies? Please advise. Have customer with a 5765 which doesn't exhibit this behavior and a 7855 that does.