Unwanted Background Colors
I had a very good question in one of my classes that is worth spending a little time on here. It concerned how to get a neutral background (black, white, gray) and how to use white balance to do that. This actually is not completely a white balance question. White balance, and setting it correctly, is important, but there are other important issues to consider as well.
First, this is going to be affected by the light on your subject and background. Sometimes the light on your subject is one thing (such as flash) while the background is another (such as flash plus ambient or existing light). In that case, it may be impossible to get a true black and keep your subject looking good. This is one reason why studio pros will typically light their subject AND their background separately so that they can guarantee the white balance will match the light on the background.
This may also be affected by the shutter speed used when you are using flash as shutter speed has no effect on flash exposure (other than sync speed) yet it has a huge effect on ambient light. And even if you do not have a different light on the subject and background, you may have enough of an exposure difference that it affects how the background is recorded by your sensor. Again, another reason why studio pros will often light both areas separately.
Second, sometimes the best looking light on the subject (and color of light based on white balance) will not give the best light on the background, especially a neutral background. Once again, a reason for separate lights.
Third, backgrounds often reflect back light to the camera in colors that are not seen by our eyes. This is often a big problem with white backgrounds and can lead to decidedly non-neutral backgrounds. Related to this and the first point, is that sometimes backgrounds will reflect back ambient light in ways that are not easily recognized by our eyes, yet the camera definitely sees it.
The answer to this is to try some experimenting. If you have a true neutral background, you can try creating a custom white balance from it (which will make it neutral) and see how the subject is affected. Or you may have to play a bit with light on the subject and light on the background.
