I wanted to achieve quite a raw quality from the light. In addition to this I wanted to play with the light temperature to achieve a more dramatic look. This first shot was taken in an old white-walled basement; however I wanted to transform it to look like a night shot of an alleyway. Firstly I taped a ½ CTO (clour temperature orange) gel to one speedlight. This was to be the key light and would warm up the overall scene. I wanted to soften this light slightly and without having any umbrellas to hand, I simply put a plastic milk container with the end cut off over my light. This works very similarly to a Garry Fong Light Sphere at a fraction of the cost. The speedlight was also zoomed in to control the light fall off (the lower half of her legs are barely lit at all). I then used a 2nd light from behind to give that slight rim light you can see on the left hand side of her hair and face. This helps to pull our model off the background. This was just a bare light, flagged to control flare. You can see by comparison how the colour of this light differs from the key light.
On this second shot, the lighting is exactly the same and we just changed shooting angle. However by adjusting the white balance of the camera to incandescent the CTO gelled light is reading as white, while the ungelled light is coming out cold, as is the light on the wall in the background. I tend to find its better to adjust the white balance in camera; that way you see while shooting how it looks however this can also be adjusted in your RAW processing software. This is a really neat trick to get completely different looks from adding a coloured gel to your light simply by adjusting the white balance.
This final image again uses a similar light set up, however the speedlights are being balanced with the ambient light this time. I wanted to shoot at an aperture of 2.8, so I got an exposure for that setting. I then underexposed the shutter speed by 2 stops in order to darken the scene a little. I then used 2 VALs (voice activated light stands), one holding the CTO gelled light to camera left for the key light, the 2nd holding the bare speedlight to camera right to give that hard rim light. The 2nd light is at about ½ the power ration to the key light.
As you can see, manipulating the temperature of light can have a huge effect on the final image. I hope this has given you some ideas to start using gels on your lights.