The basic issue: screens emit light, printed photos reflect light, so color and brightness that you see on the screen may turn out different in a print. Also, different computer screens (even different operating systems) vary in their display properties, so colors might not be consistent from one machine to another.
For example, when I print a photo at a local location, it will typically turn out a little darker and a little less saturated than it looks on my screen. When my photos (edited on OS X) get projected on a Windows PC at a photo club competition, I know the color balance and brightness will be a little different. And even if I just transfer photos from the iMac to the Powerbook to do a slideshow, the max brightness on the Powerbook still seems dingy by comparison. (Curse this bright iMac screen! ;-)
Old-school photographers often advise a complicated procedure to calibrate your monitor so that it's "accurate" -- the idea being that you'll edit the color balance, saturation, brightness, and so on so that it looks right on the screen, and will reproduce accurately when printed. That's the theory, anyway.
However, there are so many variables to consider that you can drive yourself crazy -- what "color space" is the photo using in Photoshop, what kind of printer are you printing on, what are all the other print settings in Photoshop, how long has it been since you calibrated your monitor because monitors "drift" over time!
And then you transfer the pictures to a computer that wasn't calibrated, or was calibrated differently, or you print on a different printer or type of paper, and once again you don't like the way it looks.
My advice? Follow only the simplest procedures that give the most bang for the buck:
- Do a one-time calibration with the simplest, cheapest method. Perhaps borrow a ColorVision Spyder and choose the automated mode where the device rests against the monitor for a few minutes and you don't have to do anything. Or on OS X, use System Preferences -> Displays -> Color -> Calibrate.
- Change your display's "Gamma" setting if your images are consistently too dark or too light when printed or on another computer. On OS X, you might find that your images are too dark when displayed on a PC or printed. In the System Preferences ... Calibrate procedure mentioned above, you can set up an alternative profile with a different (darker) gamma value, and switch to it at the drop of a hat. That darker monitor setting will "fool" you into brightening your pictures more than you normally would, which will give better results in print or on Windows PCs.
- If you can get a printer profile for your exact printer or your favorite online service, why not. For example, the Dry Creek Photo site has color profiles for many photo labs across the country, including every Costco. You can select the printer profile when printing in Photoshop. (Select Print -> Color Management -> Photoshop Manages Colors to enable the Printer Profile list.)
- When you think you have something close, run a bunch of test prints with variations of overall saturation, brightness, etc. Include a caption on each print to say which is which! These days, you can get 8x10s for only a few dollars each. Figure out how much your time is worth to sidestep hours of fiddling with printer settings.
- If you find you need to boost brightness by 10%, lower red saturation by 5%, etc. for every picture to get it to display properly on Uncle Bob's laptop or print on your local photo lab's professional printer, set up a Photoshop action, save a set of layers, or find some other way to apply that action to a whole set of photos at once. (Leaving the originals unchanged.) You might display photos in so many different contexts, no one color setting will work perfectly for them all.