Your digital camera has a feature called Auto White Balance (AWB) that tries to measure and adjust for these differences in light color so things that appear white to your eye are white in your photo. Modern cameras do an amazing job and get this right most of the time, however they can be fooled.
If a scene is dominated by one color the auto white balance function can be fooled into assuming that the light has a lot of that color in it. This can result in an incorrect white balance and an undesirable color cast in your photo. The two photos below show one case where the camera was fooled.
|Auto White Balance|
|Daylight White Balance|
About 3/4 of this photo is dominated by a strong yellow color from the field of Canola (Rapeseed). In auto white balance mode the camera senses all this yellow and thinks the color temperature of the light is warmer than it really is. The camera makes an automatic adjustment (the auto port of AWB) to make the photo cooler. As a result, the photographer's skin has a sickly blue color.
In fact, the photo was taken on a bright sunny day around mid-day and the light was cooler (bluer) than the camera thought. In the second photo I changed the white balance from auto to Daylight. If you compare the two photos you can see that the photographer's skin tone in the second photo is much more natural.
So, when do you rely on Auto White Balance and when do you go to manual white balance? Here's what I do. Most of the time I don't care about the white balance setting on my camera because I almost always take my photos in RAW format rather than JPEG. JPEG format has the camera's color adjustments applied, but RAW does not. With a RAW file I can correct the white balance when editing the photos on my computer. In fact, this is how I created the two example photos. I was able to change the white balance setting on the RAW file to create the two different versions. When shooting in JPEG mode it is very difficult to correct the white balance once the photo is taken.
When shooting in JPEG mode I will be aware of the color cast of the photo I'm about to take and decide if I need to override the Auto White Balance. As I said, most cameras do a pretty good job on auto mode so this doesn't happen often.
Devices such as Expo Discs can be used to determine the color temperature of the light and creating custom white balance settings. This is a more advanced topic that may get covered in a future post.