As anyone who has seen a rainbow knows, light comes in different colors, but what might not be as obvious is the subtle color differences that exist in ambient lighting. Ambient lighting is the natural light that already exists in the scene. Incandescent lightbulbs give off a warm relaxing orangish glow, mid-day sunlight is closer to white, and fluorescent lights provide a slight greenish tint. This isn’t always obvious because our eyes automatically adjust and correct for the color of light, making it appear white. This is similar what your digital camera does when it is set to AWB or Auto White Balance mode.
Your cameras white balance is a feature that exists in any digital camera which allows it to adjust various colors of light to make them appear white. It can be set to automatically make this adjustment or it can be configured manually using a preset or a color temperature.
The color temperature of light is measured in Kelvin. This is a unit of measurement for temperature, that without delving into unnecessarily complex physics, is based on the color an object will emit as you heat it up. A good example is a flame or a heated piece of metal, as it starts getting hot it glows red, then orange, yellow, white, and then blue.
The color temperature scale in photography typically goes from 2000K to 9000K. Color temperatures over 5000 K are called "cool colors" (bluish white), while lower color temperatures (2700–3000 K) are called "warm colors" (yellowish white through red). This can sometimes be confusing, since one would expect a higher temperature color to be warmer, and a lower temperature color to be cooler. However, that is not the case, and one convenient way to remember this is that incandescent lights emit a color around 3500K and tend to physically feel warm.
When relying on a single light source for your photograph, the Auto White Balance mode will often do a fairly good job, and any residual error can be corrected in post without any degradation to the image. However, manual white balance mode has a distinct advantage in that it improves the awareness of the photographer as to what color temperature the camera has chosen. This improved awareness allows the photographer to compare the image on the camera to the actual scene and ensure the image captured accurately reflects the moment. Accurately capturing the warmness in a photograph can greatly improve the feeling the moment conveyed which inspired the photo in the first place.
This chart shows common white balance presets you can expect to find on your digital camera. Each preset corresponds to a specific color temperature as shown in the table.
If a photographer were to take in a picture in a room lit with incandescent light bulbs, emitting light with a color temperature of 3000K, they would need to set their camera to Auto WB, Tungsten, or to 3000K. With any of these three settings, the light in the photograph would appear white, similar to how an observer would see it in person, since their eye would auto correct for the white balance.
If the photographer were to set the camera’s white balance incorrectly the image would have a color cast. If the temperature is set too low, the image would have a blue cast and appear cooler. If the temperature is set too high, it would appear with an orange cast and appear warmer. This is often done deliberately as a creative expression to warm up or cool down an image.
A simple approach to developing proficiency with setting white balance is to practice guessing the color temperature of each scene you encounter. Manually set the temperature on your camera in Kelvin and then take a picture. Observe any color cast in the photo and make appropriate corrections. Within a couple of practice you will be develop a skill to quickly and accurately read the color of light.
In the mean time a great tool to assist in accurately setting the color temperature is the ExpoDisc. This device automatically sets the camera’s white balance with better accuracy than your cameras auto white balance feature. Just don’t allow this this tool to become a crutch.