Reproducing printed colours
Printing is not an exact science, it is far from perfect. Unfortunately, even though the master file is the same file printed from each and every time, there will always be slight variations to the outcome, especially in the case of digital print (cheapest method).
An example of when this slight alteration may occur is when the client runs out of a piece of material and asks for a reprint - even though the same printer is used, the same machinery, and the same master design file, you can not expect to have the exact same colours come back.