Hello and welcome to Just Answer. My name is XXXXX XXXXX I will be assisting you today. Testing with another known good toner is the best way to determine this. On some Xerox color printers the colors are not keyed to their individual slots. Meaning you may be able to take another color, magenta for instance, and put it in the black slot and do a black only test. If the toner is the culprit a black test page should print out fine, except in all magenta. If it comes out light, then there is a problem with another part of the printer. You could also purchase a new genuine Xerox toner and use that for the test.