iPhoto 5 first introduced tools for manually adjusting color saturation, temperature, and tint. Why did it take Apple so long to add these tools? Color correction of any sort is devilishly difficult to do right (and as you may have noticed, even the automatic Enhance tool can get it wrong at times). Color correction suffers from two basic problems: the fact that color is highly perceptual and the fact that different devices render color in different ways.
Everyone sees color in different ways. My wife and I, for instance, frequently disagree on whether a given color is green or blue, and the fact that my opinion generally seems to match what others think as well doesn't change the fact that she is perceiving a different color. Add that to the fact that at least 10 percent of the population suffers from some level of color blindness.
The conditions in which color is perceived also make a huge difference, as you've probably realized if you ever purchased a shirt in a store lit with fluorescent lights and were surprised by how the shirt looked when you tried it on at home under incandescent lights. Similarly, when painting a room, you have to consider how the color will look in sunlight during the day and with artificial lighting at night. The differences can be striking.
The lesson here is that you cannot define color objectivelythere is no right answer. Always keep that in mind and it will remove some of the stress about achieving the "perfect" color in your photos.
Digital cameras, computer monitors, inkjet printers, and commercial photo processing equipment all use different methods of rendering color. Even with monitors, there's little common ground between normal CRT-based monitors and the increasingly popular LCD flat-panel monitors.
When you take a picture with your camera, look at it on your Mac, print a copy on your printer, and order a large print of the image from Kodak, you would like the colors in the image to match closely at each step. The engineers designing these devices have managed to make the color produced by each one match fairly well, but not perfectly. Here's how it works.
Imagine a three-dimensional graph, with the X-, Y-, and Z-axes representing the amount of red, green, and blue in every possible color. (Don't worry about this turning technical; that's as bad as it gets.) Now imagine an amorphous blob in the graph that represents the specific set of colors any given device can capture (for digital cameras) or display (for monitors or printers). That blob is called the gamut, and every device has a gamut that's at least slightly different.
The problem with matching color across completely different devices is that each device can render only colors in its gamut. When a color, say a specific light green, falls into an area where there's overlap between gamuts, each device does the right thing and renders the exact same light green. However, when a color falls outside the set of colors a device can render, it's a problem. The device cannot render a color outside its gamut, so it makes an educated guess about what color to render instead.
Many efforts have been made to address this problem, but the one you're most likely to have heard about, being a Mac user, is Apple's ColorSync technology. The particular approach it uses to make educated guesses about which colors to render on different devices is immaterial; suffice to say that its goal is consistency. In theory, if you have chosen or set up a ColorSync profile for your monitor and your printer, for instance, it should help ensure that the colors you see on your monitor match those printed by your printer.
Without getting into too many details, you can calibrate your monitor by choosing System Preferences from the Apple menu, clicking the Displays preference pane, clicking the Color tab, and clicking the Calibrate button to run and work through the Display Calibrator Assistant. Then, when you're printing, look for a ColorSync setting in the Color Management panel of the Print dialog. Whether it's present or not depends on your printer driver, but if the setting is present at all, it's usually the default. That's all there is to basic use of ColorSync, and on the whole, it works pretty well.
You may not be limited to ColorSync's educated guesses about how to render color (my Epson's Photo-Realistic mode sometimes produces better results), and in fact, none of the commercial photo processing companies, including Kodak, use it. Why not? Two reasons.
First, photos are displayed on monitors and on paper totally differently. Monitors emitlight, causing photos to be extremely bright. Paper reflectslight, so unless you shine a floodlight on a photo, you can't come close to the amount of light emanating from a monitor.
Second, color is highly perceptual, and Kodak and other photography companies have done incredible amounts of research to determine not so much how to match colors exactly, but how to print photographs that meet people's expectations.
In the end, the problem of matching color perfectly among devices is just too hard. Even with technologies like ColorSync, the differences between a photo on a light-emitting monitor and light-reflecting paper mean that the photo processing companies have a better chance of satisfying customers if they concentrate more on producing a photograph that looks desirable than on matching colors perfectly in an imperfect world where everyone sees color differently.
Should You Correct Colors?
Color correction is complex, and the necessary tools are also usually complex. Apple did a fairly good job with giving iPhoto basic color-correction tools in the Adjust window, and many photos can be improved with judicious color correction. Of course, iPhoto's tools are still limited in comparison to those in programs like Adobe Photoshop; in particular, iPhoto's tools always affect the entire image, rather than letting you select a portion of the image to correct.
Now that you know how hard it is to achieve reliable, predictable results, should you color correct your photos using iPhoto or another program? It depends on how much you want to play. For those who don't like to fuss, don't bother. If you like fiddling with your photos so you can make them just right, go ahead. And for the majority of us who fall between those two poles, I recommend doing manual color correction only on those images you like the most and that will benefit from it the most. Remember, the "right" color is the one that looks right to you.