Check to make sure that the printing press does not practice gang run printing. Gang run printing refers to the process of running multiple print jobs on the same press sheet. Its intention is to spread initial production costs across several printing jobs, thus defraying the cost to the individual. But this process has limitations, specifically the inability to control the ink density of nearby images. Therefore, there is no guarantee that your color will match the file that you have submitted. The colors will be average, and there is no guarantee against color variations within your job.
Sources
science.howstuffworks.com
http://www.webopedia.com/TERM/O/offset_printing.html
http://en.wikipedia.org/wiki/Offset_printing
http://www.pneac.org/printprocesses/lithography/
http://desktoppub.about.com/od/printingfinishing/f/offset_desktop.htm
www.theprinters.com
www.bestprintingonline.com