If you are having a problem with colour, the whites going yellow, it may be because you're shooting using incandescent light (the same cast may be there when you use black backgrounds, it's just not so apparent). If this is the case, you should be able to fix it to an extent using the white balance control on your camera.
There is an explanation of why this happens, and how to fix it, here...
http://www.photoxels.com/tutorial_white-balance.html
If this happens when you're shooting in natural light then it's probably something else.