One of my favorite portrait techniques is to add an orange gel to my flash with my white balance set to incandescent. Daylight turns blue with an incandescent white balance, and your flash also will be blue, since flash is close to a daylight white balance. The trick is to add an orange gel to your flash, which counters the incandescent white balance, resulting in a neutral daylight balance to anything the flash illuminates. This is a great technique to add mood and tension to an image and works especially well with cloudy skies.
There are other examples of counter-filtration flash. Sometimes, shooting indoors, the room is illuminated by fluorescent lights. To fix this problem, add a green gel to your flash to match the color output of the overhead fluorescent lights. Set your white balance to fluorescent, and you’re ready to go. All the light in your images should be white since the light sources are color-corrected to fluorescent. The Rosco swatchbook will show you the gel you need to color-correct the light source you have to balance. Take a shot, and if the color looks slightly off, try another tone of gel to better color-correct your light source.
If color correction sounds a little too precise for your tastes, try picking any gel you like and adding it to your flash. My favorite task when I’m using TTL flash is thumbing through the colors in my gel swatchbook, picking a color I like, and gaffer-taping it to my flash. No science involved. Maybe you’re going for a high-tech look, and adding red to a science lab gives your image the right look.
I once taught a photo workshop in the Czech Republic where we photographed a famous bone chapel. The walls of this chapel are decorated with the bones of 40,000 parishioners, definitely a moody place! I wanted to enhance the mysterious nature of this chapel, so I added blue and red gels to my flashes to get the right effect.
Take the laundromat image. My idea was to photograph a woman in a laundromat, but with a wild, crazy look. To capture the right atmosphere, I needed to add lots of color. I used six TTL flashes in the dryers with blue gels, two larger flashes on the model using red gels and another large flash in the background using a green gel. All the flashes were triggered using radio slaves. Wow! Some color combinations work better than others. Creating warm light on a cool blue background is very effective.