Can you tell which of the above pictures was from using the camera's automatic white balance and which was from using custom white balance? If you guessed that the one on the left was from the camera's automatic white balance, you got it right. When it comes to white balance the camera gets it right most of the time and this is usually when you're taking pictures outside. When you're indoor, like in this example, the artificial light creates a color cast. Your brain, using references (what you know to be gray or white) will automatically adjusts the color you see to what you know to be correct. The camera, which doesn't have such a reference, will try to its best capture the color. So it's best to either use custom white balance or some of the camera's built in white balance (most camera has tungsten, cloudy, shade, etc) to offset color casts than to rely solely on automatic white balance.
