The Lifecycle Assessments that we have run on our printing products show that there is a negligible difference between B&W and color printing.  The basic reason is that the cartridges themselves have virtually the same construction, so there is little if any difference between them.  Color laser printers have an image transfer blanket that is not found in B&W printers, this adds a minimal increase to the environmental footprint, but it's insignificant on a per page basis.