Or old ink which has dried or congealed. A PC just sends info to the printer which in turn prints based on that data. If the printer has different colors, then the problem, I would think, is with the printer, ink, or carriage.
It should have a "toolbox" or something for diagnostics. Cleaning the print heads is a good idea. This will push ink out at higher pressure to attempt to get rid of dried or stuck-on ink residue.
This may be of help http://forum.support.xerox.com/