Well its not that simple, but its basically what they do with digital cameras when they perform whats called colour balance or white balance, you basically assume that the schene that the camera is looking at is on average 'grey' and you look at the output of each of the three colour channels and you determine what you need to make to make each color channel output the same. Its sort of way of getting a repeatable kind of calibrated image, there are a lot of complicated isssues that need to be resolved when you come to implement it.
The other way is to calibrate your sensor with a light box which produces a known colour temperature, but they are expensive and you have to do it with each sensor.