To implement a Fixed White Balancing Algorithm, convention is to follow the following steps:

1. Take an image of a sample white region of the picture.
2. That is, ensure that the median of the region is less than the maximum value a pixel can take.
3. Take either median of each of the red, green and blue values in the sample white region.
4. Multiply the red and blue pixels by a gain value such that the median of the red and blue planes is the same as the green plane

Note: The black level should be set to 0.

Here is a good pdf on how to do color balancing:
http://www.stanford.edu/class/ee392b/handouts/color.pdf

Here are general websites for color balancing:
http://www.cs.sfu.ca/~colour/research/
http://ise.stanford.edu/class/psych221/projects/.../main.html
http://ise.stanford.edu/class/psych221/projects/98/siliconvision/