Question about WB

Okay,

I'm just obsessivly thinking, how do custom white balances work. I understand how to set them up and all, however I don't understand how the camera uses the CWB to affect the photo.

Can anybody help me understand this?

All WB does it say "this is white". Custom white balance just sets the temperature differently from the presets. It's easier to understand once you read a book on color, take a look at the color book in my recommended books thread, linked from the FAQ thread in my sig.

Oh, fair enough.
So what if you had a gradient of red to white as a CWB? The colours of red are reconised as white and white as white, so this would make the shade of red and white appear white?
Does this make sence?

CWB is just a single number, a color temperature K that defines the color of the light. If you shoot a gradient as your CWB weird things will happen, and it won't be accurate. What you shoot has to be neutral white or grey. CWB is the same as setting the color temperature by number in your RAW converter.

Color we see is the result of reflected light from the object. The color of the reflected light is determined by the colors in the spectrum that hit the object.

Custom WB take a known surface structure (a white or grey card, that is a card that equally reflects all colors from its surface) and measures the actual RGB values that it "sees" reflected. It then says that that RGB combination should be interpreted as white (all RGB values equal).

If the surface Custom WB sees reflects excessive red perhaps, then the camera/software knows to remove that amount of red from everything it sees to make everything the "right" color. So if you use a colored card, that is a card that does not reflect equal RGB values to WB from, you will screw things up royally.

Anything in the picture that should have equal RGB values can work for custom WB.

Generally, that's true. BUT you can use a colored card to act as a "filter" to skew the colors in the direction that you want them to go in the opposite direction. A magenta card will skew green; Blue will go yellow, etc.

Regarding WB generally, reality isn't everything. Would we try to make a sunset neutral? The "Correct" WB may not be the "Right" WB for a image. Remember, you're the judge of your own image. We used to use gels to add "romance" to our car shots for ad agencies.