Due to the complex nature of reproducing a full spectrum of colors by blending 4 colors of ink (Cyan, Magenta, Yellow, and Black), all 4-color printing is subject to some small degree of color variance. This means that specific colors may look slightly different from the beginning of a run to the end of the run, and differences become more pronouned between different runs.
4-color offset presses are able to reproduce colors far more consistently than digital or print-on-demand presses, but the blending of inks means that there will always be some variance. This variance is most noticeable between different runs of the same project, as in the photo below:
For individual colors that need to be exactly the same every time (e.g. a corporate logo), the color can be guaranteed by the use of a spot color ink. In this process, instead of using 4 separate colors of inks applied by 4 printing plates, the liquid ink is mixed separately until it exactly matches the desired color, after which it’s applied via a separate printing plate designed to apply only this single color. The specific color to be matched is most commonly specified using the Pantone Color Matching System.