Light Colour Temperature

The reason that pictures turn out with a yellow/orange cast in incandescent (tungsten) lighting and bluish in fluorescent lighting is because light has a colour temperature.

A low colour temperature shifts light toward the red; a high colour temperature shifts light toward the blue. Different light sources emit light at different colour temperatures, and thus the colour cast.


By using an orange or blue filter, we absorb the orange and blue light to correct for the "imbalance" -- the net effect is a shift in the colour temperature.

In digital photography, we can simply tell the image sensor to do that colour shift for us. But how do we know in which direction of the colour temperature to shift, and by how much?
Manual White Balance.

This is where the concept of "White Balance" comes in. If we can tell the camera, which object in the room is white and supposed to come out white in the picture, the camera can calculate the difference between the current colour temperature of that object and the correct colour temperature of a white object. And then shift all colours by that difference.
Most advanced digital cameras therefore provide the feature to manually set the white balance.
By pointing the camera at a white or grey card (angled so that it is reflecting light from the room) as a neutral reference, filling the screen completely with it, then pressing the White Balance button (or set it in the menu), the camera does its WB calculation.
From then on, any picture taken will have its colour temperature shifted appropriately. It's quite simple, really, and you should not be afraid to try it out and see your indoors pictures improve considerably (assuming there is enough light for correct exposure).
[A "neutral" grey is 18% grey and will reflect all colours equally.]

Caution:
Ensure the card is not in shadows, but illuminated by the artificial light in the room.

If you are bouncing light off the walls, ensure the card is reflecting the bounced light

Beware of mixing artificial lights -- in this case, you might want to use RAW and adjust in post processing for each light

Beware of fluorescent light: since fluorescent light does not contain all the spectrum of light, you may obtain unexpected results.

David B Flynn.