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Auto white balance sounds like it solves every issue regarding colors in your pictures, but it doesn't. For example, when you shoot at sunset or sunrise, AWB wants to 'correct' the golden tones that we love so much when the sun is close to the horizon. It desaturates the yellow and red portion of the spectrum, and the colors look weak and disappointing. By contrast, if you shoot with daylight white balance, you will capture the colors you see. The yellows, reds, and oranges will be saturated and dramatic.
It is true that if you shoot in RAW mode (as you should be doing for maximum image quality and the ability to tweak your pictures in many ways), you can correct the color balance using the 'temperature' slider in Adobe Camera Raw or Lightroom. However, who wants to correct every picture taken at sunrise and sunset? Use daylight WB, and you won't have to deal with this time-consuming problem later.
If you shoot in JPEG mode, you can't correct an incorrect color balance such that it looks just like the colors you saw with your eyes. You can use Photoshop to approximate what you saw, but it won't be completely natural looking.
Golden colors in the sky and on landscapes imbue images with warmth, beauty, and artistry. The picture of the Eastern Sierras, below, is an example of a scene where desaturation of color due to using auto white balance would be detrimental to the image.
Whether you are shooting into the sun or have the sun at your back, it's important to use daylight white balance. The photos below of Ogrodzieniec Castle in Poland, and the landscape surrounding the castle, are an example. Note the golden hue on the ancient architecture, and then when I turn 180 degrees to shoot into the sun, the intense colors (and the low angled sunlight) make the scene beautiful.
When shooting in deep shade, the daylight white balance setting will produce bluish tones. Some photographers don't like this, but I like the cool tones of an overcast sky or shade. If I want to correct the blue and make the image warmer (i.e. more yellowish), that's easy to do by using the 'temperature' slider in ACR or Lightroom. The reason I don't switch to cloudy white balance is because it's one more thing to think about and slow me down when I'm shooting. I prefer to concentrate on the composition, the light, and the subject matter.
In the outdoor fashion shot below, the blue tones on the Spanish mission in Arizona are pronounced, but in this case I think that makes this image more compelling. In fact, those colors complement the model's dress.
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