Earlier this week we looked at ways make basic contrast fixes in iPhoto's histogram. Now that you know a bit about how the histogram works, you can tackle one of the most challenging image problems to solve: bad color. If you shoot without a flash while in the shade, your photos will likely take on a bluish tone. Shooting indoors next to a lamp can produce yellowish images. But it's not always clear what needs to be done to remove unwanted colors. Here, too, the Levels histogram can help.
Automate Color Corrections
Before you start fiddling with all those sliders and buttons, see if iPhoto will perform the color corrections for you. Although iPhoto's Enhance button isn't always the one-click solution to troubled photos that it promises to be, it does make for a good starting point if you're not sure what to do with your photo. When you click on the Enhance button, iPhoto shows you what changes were made in the Adjust palette (this is an improvement over previous versions, which offered no way to see exactly what was done). From there you can fine-tune the sliders to get the exact results you want.
For another fast way to adjust a photo's colors, click on the small eyedropper icon to the left of the Tint slider. This is the Auto Gray Balance tool. Search your picture for an area that should be a medium to dark gray, and then click on it. iPhoto will automatically adjust the Tint and Temperature for the entire picture. If you don't like the results, press Command-Z. You may need to try a few different gray areas to find the results you like the best.
Quick Tip: To quickly compare the changed image with the original image, press the shift key.
Adjust Colors Manually
The image on your computer screen is made using three different color channels--red, green, and blue. When combined the right way, these three colors create all the other colors and grays you see. When combined at full strength, they produce white.
iPhoto's Levels histogram displays three sets of colored bars--one for each color channel. By examining how these color channels overlap, you can get a sense of where color problems may exist.
Consider, for example, the picture of a tunnel. If you look at the original's histogram, you'll notice that the three channels barely overlap--especially in the highlights. Since you need all three colors to produce a true white, the brightest areas of the image appear to be yellow.
To change the alignment of an image's color channels, use the Temperature slider. Move the slider to the right to make your image warmer (shifted more toward red) or to the left to make it cooler (shifted more toward blue). In the case of the tunnel picture, you'd move the slider to the blue side until all three channels ended at roughly the same place on the histogram--producing a more accurate white.
Once you have the channels lined up, you may discover additional color problems. In this case, the image became a little too blue overall. To fix this, move the Tint slider to warm up the image while preserving the tonal distribution (so whites stay white). The result is much truer color and a shapelier histogram.
Fix Dull Colors
Want colors that really pop? Dragging the Adjust palette's Saturation slider to the right will make your photo's colors more intense.
Most digital photos look best with a little extra saturation. The only problem is that bumping up saturation can make flesh tones look unnatural. To keep this from happening, iPhoto now offers an option to minimize the effect of the Saturation slider on skin tones in a photo; if iPhoto's Faces feature recognizes that a photo has a face--or you've added a missing face to a photo--the Saturation control automatically has the Avoid Saturating The Skin Tones box checked.
Go to Grayscale
If the colors problems in your photo can't easily be solved with these techniques, your best option may be to eliminate the color completely. iPhoto actually offers a couple of ways to create a black-and-white photo. The first is to simply drag the Adjust palette's Saturation slider all the way to the left (you'll need to make sure the Avoid Saturating The Skin Tones option is deselected). However, you often get better results by opening the Effects palette and clicking on the B&W button. You can also use the Effects palette to fine-tune your black-and-white image. Clicking on Boost Color will give you slightly richer, warmer tones, while clicking on Fade Color will give the image a cooler, slightly bluish tone. Click on the buttons multiple times to exaggerate the effect.
[Macworld senior contributor Ben Long is the author of Complete Digital Photography, fourth edition (Charles River Media, 2007). More of Ben's work can be found at Complete Digital Photography.]
This story, "Correcting Color Problems in IPhoto" was originally published by Macworld.