If you think about it, between your monitor, printer, and scanner, the different components of your color management system (CMS) usually, without proper calibration, define and display the same colors differently. In fact, it’s quite common for various colors to “shift” to other colors between two pieces of equipment. Hence, to get the best possible results, you must keep your equipment calibrated, so that each component defines the same colors the same as the others.
I showed you how to calibrate your monitor to your printer, so that these two devices define colors accurately between them, a few months ago. It’s just as important that your monitor and your scannerdefine and display colors accurately between themselves, too. Otherwise, the blues you scan might shift to purples and the reds to dark maroon.
Before going into this, though, I should point out that it’s usually more efficient to calibrate all three devices, the scanner, monitor, and printer, at the same time. You can do that easily enough by combining the procedure listed in this About.com “Calibrating Your Monitor to Your Printer” article with the procedure laid out here.