Usually it's funny test new ways to obtain some results.
I've two Canon 580EX flashes and I'm already comfortable with coloured filters which fit under the plastic flap. Anyway since I though up a way to make a filter from a commonly used tool, I wish to show you this brain-child.

Step 1:

I bought these silicone molds for about 1$ each. They're of the right size to fit on a common flash for photography. And furthermore silicone lets light passing through with a nice coloured effect.

Step 2:

As you see from the picture, I noticed that cyan container, although it's indeed lighter than the blue one, lets passing less light than the others. So it's not much useful for our purpose. So I decided to use the red, blue and orange molds to create three coloured filters.
You'll see as these filters can be handy attached to any flash.

Step 3:

Cut the molds with the same width of the flash head, and draw a rounded shape on the shorter sides of the front surface, cut following the curve.
Do that for all three containers and keep the silicone scraps for other projects.

Step 4:

Now find an elastic of the right size and fit your new filter on the flash... not much to say about that ;-)

Step 5:

These are your new coloured filters, you can roll them up to keep them in your pocket, believe me, they're very handy, and they don't crumple as plastic sheets.

Step 6:

Test them with a fixed white balance (I used cloudy weather) and enjoy new effect. In this way you can use the second flash as backlight and change the colour of the background in a portrait session.