Either you set the print policy to BW CWIS > Properties > Services > Printing > General
Or you have an accounting system for color print (Apple doesn't support this) so it prints BW instead of failing the job, those 2 options are here.
CWIS > Properties > Login/Permisions/Accounting . User Permissions > User Permissions Roles > Edit
If option 2 is the issue, you may be able to set the iPhones/iPads as IPP exceptions and cludge it into working, but you definitely won't be able to track the user.
We have a few WC7855i copiers, they have airprint enabled and printing does work.
Problem is nothing will print in colour, every job comes out in greyscale.
Apple devices are fine, they will print in colour to Dell colour printers, just not with these Xerox copiers.