Calibrating Color Images
If you are working with photos from a variety of sources you will find that they have very different characteristics: Some may appear warm, others much cooler. Daylight produces different color casts depending upon the time of day the shot was taken, and indoor photos will often be lit by a combination of artificial and natural light. Artificial lighting may be fluorescent (cool) or incandescent (warm). All of these present potential problems to the designer, because they add image-wide color casts which are not wanted.
It is tempting to ask the printer to take care of all this for you, but that is a very costly way to fix something that you could quite easily do yourself. With experience, you will never have ...