You are probably having trouble because you are making it harder for yourself than it is.
Your brain lies to you, the camera doesn't
The basic fact is this: Light comes in different colors. This is true apart from photography, but we tend not to notice it, because our minds automatically adjust. We don't notice that colors look different in florescent light than in normal sunlight. In the same way, we tend not to notice how sloppy somebody looks or how messy the room we're standing in really is. We tune these things out normally.
Until we take a photograph. Because the camera does NOT tune things out. It doesn't clean up the room or tuck your friend's shirt in for him. And it doesn't automatically adjust the color of the light. If your camera is set to expect normal sunlight, and you shoot a photo inside at night using the light of a normal tungsten lamp, well, the colors are probably going to look "wrong."
Try it for yourself
The best way to understand this is simply to see it for yourself.
Open a book with white pages under a lamp in your living room at night. Set your camera's white balance to something inappropriate, like sunlight. Take a photo. Look at the photo on the camera's display screen. What you see won't be pretty. The white pages of the book may look orange-ish—or perhaps it will seem more yellow or more red to you, depending on your light, the actual whiteness of the book, and other factors. Anyway, it will look awful.
Now, change the in-camera WB setting to tungsten light, and take another photo. This time the white pages of the book will probably look much better, much more like what you actually see.
Finally, change the in-camera WB setting to AUTO and take a third photo. This time the photo may look even more like what you THINK you see, that is, the white page may look downright white.
At least those are the results I got just now when I did this same experiment here. On auto, the camera was actually overcompensating a little bit.
Which of the three photos is best? Well, it's probably not the one where the white balance was set wrong, to sunlight, although occasionally photographers have been known to deliberately do this sort of thing in order to achieve a special effect—say, to give a warmer appearance to a scene. The photo you will like best is probably one of the other two. Why can't we say for sure which is best? Because our minds continue to compensate even when we look at the photo on our computers (or in print).
Shoot raw + AWB and worry no more
Want to stop thinking hard about white balance? It's easy to do. Two steps:
- Set your camera to shoot raw.
- Set the camera's WB setting to Auto.
If you shoot on auto white balance, the camera will calculate what it thinks the right white balance to be and embed that info in the picture file. When you open the raw file on your computer, your software (Lightroom, Photoshop, whatever) will accept the camera's guess about the right white balance and display the photo that way. My Pentax DSLRs are generally VERY good at getting the white balance right in auto.
And if you shoot raw and the camera's guess about the white balance doesn't strike you as quite right, you have all the data necessary to modify the white balance after the fact. If you shoot jpeg, on the other hand, while you can adjust the white balance on your computer you won't have nearly the same latitude to do so as you would if you shot raw.
Two final comments
First, although white balance seems largely a digital problem, but actually, the color of light was a problem back in the film days, too. Different types of film were better suited to different types of light.
The other comment is that white balance can become a very difficult problem indeed when there are mixed light sources, say, some daylight, some light from a bright tungsten lamp, perhaps some florescent light thrown in there for good measure, and (gack) the light from a flash. This is when you will want to convert the photo to black and white. ;-)
Will