What we perceive as white light varies in color or temperature from cool blues to warm reds. Our brains are brilliant at processing these variations but an SLR needs you to set the white balance accurately.
It's important to select a custom white balance correctly to avoid ending up with unwanted color casts. So why not leave it permanently set to auto balance? Well, auto white balance has to calibrate and adjust for each frame, which means that temperature becomes inconsistent, causing a range of color casts across your photos.
The best option is to select the appropriate custom white balance for lighting conditions. Some of the common color temperatures are available as presets, represented by symbols such as a cloud or a sun. These presets are surprisingly accurate and usually provide great results. Try switching to the cloudy setting when shooting under overcast skies or to the light bulb when shooting under tungsten light.
If your SLR has pre-white balance feature taking a reading of the scene will provide even better results. The most accurate white balance possible will come from measuring lighting temperature using a light meter and selecting the appropriate K value in the white balance menu.