The problem is that these different devices "see" colours in different ways. Scanners, digital cameras and monitors typically treat colours as combinations of red, green and blue pixels (RGB), while printers usually treat colours as combinations of cyan (bluish), magenta (purplish red) and yellow, with black thrown in for contrast (CMYK - K is used for black because B might make you think of blue and we don't want to confuse people).
Even within these dissimilar colour models, though, various devices have different defaults for how much of each colour is required to make up different shades - what looks blue on your digital camera, for instance, might be decidedly greenish on your monitor. Translating between colour models, as for printing, means the computer has to work out what values of C, M, Y and K will give the same result as the combination of R, G and B on screen. It all gets very hairy.
For some years, Apple has provided a standard bit of software on all Macs called ColorSync, that enables users to "profile" their input devices such as scanners and digital cameras, and output devices such as monitors and printers, so that what looks red on one will look red on the others. Aside from graphics professionals, few users bother to delve into the depths of what ColorSync can do.
When you create a graphic file in a program that supports ColorSync, a profile is embedded within the file. The profile contains information about the equipment on which the file was created, including the lightest and darkest possible tones, and the complete range of colours that the device can support (also known as the gamut). When the file is moved to another Mac with ColorSync, this embedded profile is compared against the profile for the new Mac and ColorSync adjusts the colours so that it looks as similar as possible on the two machines. Likewise, when the file is moved to a device such as a printer, the embedded profile is compared against the device's ColorSync profile to match the colours as closely as possible.
Note: although Color-Sync gives better results than you would get without it, it is pretty much impossible to get printed output, created by mixing shades of ink, to look "exactly as it does on screen, where it is created by mixing shades of light.
Profiling: Professional users employ a variety of devices such as light meters and the like to profile the exact specifications and capabilities of their equipment. They also create multiple profiles for use in different situations, such as the way the light in the office changes from morning to afternoon. This level of precision may be necessary for people whose livelihood depends on precise colour.
Thankfully, for most of us, there is an easier way. Most devices manufactured for the Mac come with ColorSync profiles on the installation disks. When you install the drivers for these devices, the profiles should automatically install into the ColorSync Profiles folder (inside the System folder). If they aren't automatically installed, simply drag them over.
If the profiles aren't included on the discs, they may be available on the manufacturer's Web site. If you can't find a ColorSync profile, look for an International Colour Consortium (ICC) profile - it's much the same thing and will work just as well. Simply drop this into the ColorSync Profiles folder.
Next thing to do is to calibrate your monitor to ensure that the colour represented on it is as accurate as possible. You do this using the Monitors and Sound control panel.
When you first open the control panel, you have the option to adjust brightness, contrast and colour depth. You should select the maximum colour depth. Then set the brightness to a comfortable level where the border around the screen looks black and not grey, and set the contrast to the maximum level.
Next, click on the "Color" button to recalibrate your monitor. You're presented with three pop-down menus: White Point, Gamma Curve and Ambient Light.
White Point settings affect the luminescence of colour as it appears on screen. If the image work you'll be doing will mostly be viewed on screen - such as PowerPoint presentations, Web sites or QuickTime movies - select a White Point of 9300. If you'll be doing work for print, choose D50, which gives the screen a more "paper-like" quality.
Next, choose a Gamma setting. This equates to the degree of contrast the monitor can display. The default is 1.8, and this is adequate for either print work or still images viewed on a Mac. If you'll be doing digital video work, such as with iMovie, or if your work will be viewed on Wintel machines, you may like to switch to a Gamma of 2.2.
Setting Ambient Light is only really useful if you intend to change your ColorSync system profile regularly as the day changes to cope with changes in lighting conditions. If that doesn't sound like you, leave the Ambient Light setting at "none".
If you click on the "Preferences" button, you'll be given the option to recalibrate automatically every so often, or when changes are made to the system. You can most likely ignore this. You can also set the accuracy of calibration, which defaults to somewhere between "normal" and "pro". If you're doing a lot of printing, set this closer to "pro"; if you're mostly doing video, set it closer to "normal". For a combination of the two, leave it around the middle. Click OK to save changes.
Next, click Recalibrate. The Mac runs through a self-calibrating routine based on the settings you've specified. When it's finished, if you're happy just close the control panel. You'll be asked to give your new ColorSync profile a name and save it - it then becomes your default system profile. If you're not happy, you can change any of the settings and recalibrate again. Of course, you can also create multiple profiles (if, for instance, you want one profile for when you're doing video work and another for when you're printing).
The next thing to do is to open up the ColorSync control panel. This control panel is mostly there for the professionals who know the difference between "3M Matchprint", "Generic (Japan) Proofing" and other arcana, not for general home users. If you've created multiple profiles for different purposes, use this control panel to switch between system profiles. Also, you should set the "Preferred CMM" option to "Automatic" - this lets ColorSync choose the colour management module that best suits the file you're using. Otherwise, leave the control panel alone. When ColorSync encounters a profile embedded in a file, it will select the settings best suited for that file and you need never know the difference.
Once you have your ColorSync profiles in place, the next thing to do is to open the applications you use for scanning images, capturing video, manipulating images and printing. Turn on any ColorSync options that are available (look in the preferences as well as the menus for printing, exporting and saving files). If you have more than one colour management system installed (some digital camera software comes with proprietary colour software, for example) switch this software off - ColorSync hates to fight.
And there you have it. The results may not be quite right at first, but feel free to experiment with different settings and profiles. Once you've got the knack of using ColorSync, all of your graphics work will seem much more professional.