Have you tried using colored gels on your external flash units before? There are two main reasons to use them- one is to color correct, the other is for effect. 
Left: A selection of gels. Right: A gel in use.

If you were shooting colored film, you might use a filter that goes onto your lens to correct the color balance depending on your lighting situation. When shooting digital, we use white balance settings. But when you get into mixed lighting situations, for example using a flash while shooting indoors under incandescent light, it gets a bit trickier to color balance the whole image correctly. Since you are then using two different types of light sources, which have two different color temperatures, you cannot use one white balance setting. By putting a color correcting gel on your flash in this situation, a gel that is similar in color temperature to the temperature of the area you are shooting in, then you will only have to white balance for one color temperature, instead of two. The correction gel colors are: blue for outdoor shade, green for fluorescent, and orange for incandescent lighting. So, if you're shooting under incandescent lights and need to use your flash as well, then you would add an orange gel to your flash (which would end up giving off a similar color of light as the incandescent lights), and color balance for incandescent light.

1: Shooting with no flash in incandescent light, auto white balance.
2: No flash, incandescent WB.
3: With a flash, auto WB.
4: With a flash and orange correcting gel, auto WB.
5: With a flash, orange correcting gel, incandescent WB.

In the example shots above (all SOOC), it shows us that if we're not using a flash in this setting then we loose a lot of detail on the deer. When we add the flash, you can see two very different light temperatures competing with one another. As we add a color correcting gel and change our settings, we get closer to a correctly white balanced image. Adding a gel might not be an immediate fix-all, but it will help in the process.

But flash gels can be used for much more than correction, they can also be used for a variety of lighting effects. An example of when you may want to use them, is if you were shooting a sci-fi scene and needed to create a colored spotlight for an extraterrestrial character.

Below are a few other ways that colored gels can be used for effect...

* To color the entire image (below: 1 light with a gel on it)

* To add multiple colors to an image, and/or change the background color (below: 2 lights used- both with different colored gels)

* To add a colored rim or back light (below: 2 lights used- one without a gel, one with)

* To create a colored shadow (below: 2 lights used- one without a gel, one with)

* To add a pop of color to a silhouette (below: 2 lights used- one without a gel, one with)

Gels come in many different colors, and you can buy them in ready to use sets or make your own. One of my personal favorite gel sets is the Rogue Universal Gels. You get 20 gels in a carrying wallet that has little dividers in it. The gels fit easily on your flash unit with a Gel-Band (many other sets you have to add Velcro to your flash unit and the gels).

There are endless ways to be creative using colored gels, but keep in mind that since you are adding something on top of your flash, often a much darker colored gel, that you will have some f-stop loss. The loss typically ranges anywhere from 1/2 to 3 1/2 f-stops of loss. Like most things, just take some time to practice and experiment, and have fun with it!

- JF, photos © BlondeShot Creative