“Gel shifting” means shifting the color palette of a photo by using a gelled flash to change the lighting on the subject and changing the white balance to change the background colors. The term “gel shifting” isn't known by the vast majority of professional photographers. Usually this is referred to in photographer circles as “that thing where you use a gel on your flash and then change the white balance.”
The photo above is an example of gel shifting. This photo was taken using an orange gel on the flash, and changing my white balance to “incandescent” or about 2500k.
The photo below is the exact same picture as the one above, just that the white balance on the camera is set to neutral outdoor lighting. You can see in this photo that the orange gel on the flash makes the athlete look orange. With the white balance changed on the camera to make orange look white (setting the white balance to incandescent), the orange athlete looks neutral, but the background clouds which are too far away from the flash to be affected turn to blue because of the white balance change.
- 1/250 – I shot this photo using two cheap YN-560 III speed lights, which have a flash sync speed of about 1/200. I cheated slightly. Even though I was panning with the subject, there is still some blur on the athlete in this shot. It's personal preference, but I kind of don't mind it since it adds a sense of action to the shot.
- f/18 – Since I had such a slow shutter speed, I had to block out quite a bit of light or the picture would be overexposed.
- ISO 500 – Whoops! I really should have reduced my ISO to 100 and dropped down my aperture, but I goofed. A lower ISO would have given me very slightly less noise in the shot.
- Nikon D810
- Nikkor 16-35mm f/4 lens at 19mm
- White balance: incandescent
If you're ready to learn flash photography, I highly recommend you read my free “Flash Photography Crash Course” series. In those articles I take you through what it takes most photographer a year or so of flash photography to learn, and condense it down into a short series of articles. It's definitely worth a read.