Where ISO deals with the amount of light, White Balance deals with the color of light. The part of the brain that processes the information from our eyes does an excellent job of “white balancing” the light we see. We can walk from outdoors (daylight) to an office building illuminated by fluorescent lights and then into an office illuminated by tungsten bulbs and our eyes will adjust automatically. You may get a glimpse of the green colorcast from the florescent lighting or the orange colorcast from the tungsten but your brain corrects for those color casts in an instant. Your camera makes similar adjustments when it is set to Auto White Balance, and the newer cameras do a great job automatically adjusting the White Balance. You might be thinking that relying on auto anything is what we are trying to get away from. While this is true auto white balance can give you excellent results in mixed lighting conditions.
Your camera has several white balance settings to get you in the ball park but what happens when you have two or more different light sources in the same scene? This is what is called mixed lighting and Auto White Balance can be very useful in these situations. If you set the White Balance to just one of those light sources you will still get the colorcast from the other light sources. One mixed lighting situation is Night and Low Light photography in a city. I one scene you might have sodium vapor, carbon arc, neon, tungsten and fluorescent light sources.
These settings will be better than Auto when there is a color cast around that can fool the camera. When light hits an object it will reflect the color of that object back into the scene creating a color cast. For instance when you are making images in a room with brightly colored walls the light that hits those walls will reflect that color back into the room. The most precise white balance adjustment you can make is creating a Custom White Balance for what ever you are photographing.
Creating a Custom White Balance is not difficult but it takes a few steps if your camera has this option. Check your owners manuel for the specific steps you need to follow for your camera. You will need a “target” that is either middle grey or white. Once you have your camera setup to create a custom white balance, you fill the frame with the target. You then trip the shutter and the camera tells you if you were successful. Now your white balance is set for those lighting conditions and only those conditions. When the lighting changes you need to create a new custom white balance.