This photo, which I took at the Snoeck Ducaju & Zoon printing plant in Ghent, Belgium, a few years back, clearly shows the four-color printing process. The workers are cleaning the presses, and they have removed the plates. From back to front you can see the colors of conventional four-color (CMYK) printing: black (the “key,” K), cyan (C), magenta (M), and yellow (Y).
Yellow plates are made not to look quite the color of yellow ink in order to see them better. The yellow is added last, and too much can create a kind of milky fog, so adjusting the yellow is often a place to start in color correcting on press.