Although modern cameras automatically calibrate white balance, corrections are sometimes necessary.
While the human eye isn't sensitive to the color temperature of light, a camera sensor is. That's why white light doesn't always come out as white on pictures, and they can seem "cold" (too blue) or "warm" (too yellow).
Despite the fact that the eye perceives both twilight and candlelight as white, the difference in color temperature between the two will be visible in a photograph.
To correct the color temperature of light, choose the command Color > Temperature. Correct the white balance of the image by moving the slider to the right or left.
This process can be used to correct pictures taken in rooms lit by incandescent light bulbs (they'll be too "warm") or in the shade on a sunny day (with the blue sky as the only light source, they'll be too "cold").
The color temperature of light is stated in Kelvins (K). Approximate temperatures for various light sources are given below.
1,800 K - Candle flame
2,500 K - Incandescent light bulb
5,500 K - Neutral color (diffuse daylight, study lamps, camera flash)
10,000 K - Daytime sky
15,000 K - Sky at dawn or dusk