Text —especially fine text or text set at a small point size— on colored backgrounds is extremely difficult to print in register. The slightest misregistration can become noticeable as small gaps can appear between the text and the underlying colored elements.
To avoid this problem, you can specify that all black text should print on top of any colored backgrounds. This technique is known as overprinting. Overprinting preserves the legibility of the text.
You can specify overprinting only for 100 % black text because printing text of any color other than solid black over any other colored background might cause the overlapping inks to blend, which could result in unwanted color mixing.