When it comes to color, there are many factors to be aware of:

1. The Effect of Lighting – Color viewed under different lighting conditions will look different.  Another issue with lighting is called metamerism, which occurs when two color samples appear to match under a particular light source, but not another.  Similary issues can develop due the strength  and color of flourescent lights, and their age.
2. The Effect of the Surround – Adjacent colors, or the the background against which a proof is viewed can also influence color perception.  This is called the “adjacency effect” and can be minimized by always viewing samples against the same (Munsell N8 Gray) color background.
3. It’s all in the Paper – The greatest influence on color is the paper type upon which it is printed.  Subtractive color refers to using cyan, magenta, yellow, and black inks on a white paper to reproduce full color.  Correspondingly, the whiter the paper, the greater the color gamut that can be reproduced on the press.  Other factors that influence color reproduction include the amount of recycled content, the coating, and the finish of the paper.
4. Converting color to CMYK - When images are converted from RGB to CMYK, the halftone dots for the black ink are typically generated in one of two ways: Using undercolor removal (UCR) or gray compontent replacement (GCR).  This choice determines how much black is introduced into the image as well as how much cyan, magenta, and yellow ink will be printed.
5. Terminology – Describing color is difficult because it is a sensation and is often decribed in subjective terms by the viewer.  We try to describe things using consistent terminology to eliminate confusion and to help educate on the printing process.
6. Reproduction – Printing is not an “exact match”, it is a reproduction, or a likeness.  Just like a camera cannot reproduce all of the colors that the eye can see, the color gamut (or number of reproducible colors) is diminished in any reproduction.

Here are some videos with additional information discussing color provided by the Printing Industries of America.

On describing color:

On viewing color: