I get the following error when printing to HP3500n Colorlaser from a database driven program called "studio designer".  This is the what prints out:

PCL XL error
subsystem: KERNEL
Error: ColorSpaceMismatch
Operator: VendorUnique
Position: 9

I have no idea what this means.  But I have found that setting the printer to print in "monochrome" does not generate an error.  However, it is not desirable to have to make this change every time I want to print an invoice.  Can anyone offer a suggestion as to what the error is caused by and/or how to resove it?

The VendorUnique operator is a 'private' operator; the attribute values of the operator are defined by different vendors for their own proprietary purposes.

Perhaps what the error message implies is that you are using a printer driver designed for one make of printer on a different make?

Or it could just be a driver bug; check to see if a later driver version exists.

Or (as you'll find advised with lots of other posts regarding PCLXL errors), change to using a PCL5e or PostScript driver, instead of the PCL6 driver (PCLXL is the 'official' name of the PCL6 language).
That way you should not get any PCLXL errors (although you may then encounter other problems associated with the chosen alternative language).