I wil write this assuming the reader has a basic knowledge of white balance and the custom white balance feature on their camera, when time permits (and if anyone is interested!)I will expand on those themes.

This concept relies on the use of a gel on your external flash. A gel is a small piece of coloued plastic, often used in theatre lighting to colour the lights. It can be as simple as a piece of cellophane, if it's coloured and you can see through it, it can be used as a gel. I normally attach the gel to the business end of my flash with blutak. Some photos of this will be forthcoming.

I will use the examples underneath to explain the method.

In this example I used a blue gel. I had the model hold a white card, then set the camera to custom white balance. I filled the frame of the camera with the white card and then the fired a shot. The blue flash lights the card, which then tricks the camera into thinking the whole scene is lit with blue light. The camera then compensates by giving any subsequent photos a strong yellow cast, which is the opposite of blue.

This has the effect of anything within the scene that is lit by the bue flash will appear to be normal, and anything not lit by the flash, ie the background, will have a strong yellow cast

Here is the before photo - with normal lighting:

And here is the after photo, with a blue gel on the flash, the flash here is hand held off to the left of camera:

The use of different colour gels on your flash will have dramatically different effects on the background, for example the use of a yellow gel will give a blue cast, or a red gel will give a green ting to everything not lit by the flash.

Hours of fun for everyone!