Nikon D300 User Manual page 676

Back in the "good old days" we'd buy special rolls of film or
filters to meet the challenges of color casts that come from
indoor lighting, overcast days, or special situations.
The D300(S)'s method for balancing the camera to the
available light comes with the White balance (WB) controls.
How Does White Balance (WB)
Work?
Normally White balance is used to adjust the camera so that
whites are truly white and other colors are accurate under
whatever light source you are shooting. You can also use the
White balance controls to deliberately introduce color casts
into your image for interesting special effects.
Camera WB color temperatures are exactly backwards from
the Kelvin scale we learned in school for star temperatures.
Remember that a red giant star is cool, while a blue/white star
is hot. The WB color temperatures are backwards because the
WB system adds color to make up for a deficit of color in the
original light of the subject.
For instance, under a fluorescent light, there is a deficit of
blue, which makes the image appear greenish yellow. When
blue is added, the image is balanced to a more normal
appearance.
Another example might be shooting on a cloudy, overcast
day. The ambient light could cause the image to look bluish if
left unadjusted. The White balance control in your camera
sees the cool color temperature and adds some red to warm
676