Nikon D300 Complete Manual page 316

color temperature values than Nikon suggests for most
indoor lighting (e.g. Incandescent a1 or a2).
The D300 detects automatic white balance via two different
systems: (1) via the 1005-pixel sensor in the viewfinder; and
(2) via the main imaging sensor itself. Nikon doesn't reveal
how these two systems interact, but a reasonable guess is that
the viewfinder sensor is primarily used to estimate white
balance so that exposure can be set properly. The EXPEED
system almost certainly handles automatic white balance from
image data.
Let's look at color temperature in action. Since color
temperature for daylight was originally determined on the
Washington Mall, let's go there for our test. Below you'll find
a photo taken late in the day (in late April) of the Lincoln
Memorial. I've tweaked this photo a bit to saturate the colors
and tone down the sky (which also has a graduated neutral
density filter holding it back), but if you were standing next to
me at the time, this would be pretty close to what you saw:
The left portion of the monument is the area we'll work with
(though note the slight orange areas under the eaves in the
Thom Hogan's Complete Guide to the Nikon D300
V1.02
Page 316