Sometimes people ask for last minute colour adjustments on press. While some fine-tuning can sometimes be made, there may be unintended consequences. Changing one colour will have an impact on the entire press sheet.
Why? Because ink is applied to the sheet of paper in the same way that paint is applied by a paint roller. The density of the ink on the sheet must be consistent from top to bottom and from side to side. If one ink density is adjusted—for example, to reduce the red in the skin-tone in one photo, that change can affect the other images on the entire press sheet. And that one press sheet may have eight pages of your booklet printed on it. (see question 1-A, above).
These and other questions are covered in more depth in the Graphic Arts Production Handbook “Pocket Pal”, which can be purchased through 1-800-854-3212.