The Tint and Temperature controls go hand-in-hand with the fundamental concepts for light and photography. And luckily, they're not hard to learn. To use these controls effectively, all you need to know is what impact each has on the classic lighting issues each was built to address.
In a nutshell, the Tint slider lets you control the amount of red or green tones in your image (just like the Tint control on your TV). Move the slider to the left to add more red and lessen the amount of green. Move it toward the right to add more green and lessen the amount of red. This slider comes in handy when you're trying to make skin tones look good, want to balance out scenery that contains a lot of plants, or forgot to use your camera's fluorescent white balance setting.
The Temperature slider is kind of like a "white balance" corrector in that it allows you to introduce more warmth (more orange tones) or coldness (more blue tones) to an image. For example, if you shot an image using the camera's Auto white balance and your prized polar bear picture is looking mighty blue, move the Temperature slider to the right to warm up the image, or you can move it left to give the image even more of an Arctic chill.
Have you ever taken a picture in a room lit by fluorescent light and sunlight pouring in through an open window, and then your flash went off too? You might notice that the color in the resulting image looks a little funny. This is because each one of those lighting sources emphasizes a different color in the visible light spectrum (like the colors in a rainbow), and they're all mixed up in your photo. iPhoto's Temperature and Tint sliders allow you to correct for that.
To see how this works first hand, change the white balance setting on your camera from Auto to Daylight. Then get a sheet of white paper, and take a picture of it in each of these conditions:
Once you download the pictures to your computer, you should see these results:
Tungsten, sunlight, and flash are all light sources that represent a full light spectrum, but with emphasis at different points. Because the spectrum is complete and continuous, you can move iPhoto's Temperature slider to remove a warm or cold cast, revealing the full range of color underneath.
However, most fluorescent lights, as well as certain types of outdoor lighting, have gaps in the light spectrum they emit. This is why images taken under these types of lights can't be color corrected effectively using the Temperature slider, which only works its magic on the warm-cold continuum. However, moving the Tint slider can remove a green or purple cast from the fluorescent light. But because the fluorescent spectrum is incomplete, a Tint correction generally can't achieve the look of natural light in the way that a temperature correction can, because certain colors are missing from the light, and you can't reveal what's not there. For this reason, it often helps to mix flash with fluorescent light, to round out the spectrum.