When working with a flash a photographer has to watch out for different colors of light. Light has a color. That color is measured in degreed Kelvin (K). Sunlight is a different temperature than florescent light and florescent light is a different color than a flash. To correct the flash light temperature gels are used.
This is a picture with my camera set to auto white balance.
The light looks orange. The next picture I set the color temperature in my camera to incandescent light color temperature.
The light now looks more balanced and the right color. Now lets say that I wanted to fill the area around the light with a flash.
The light stays mostly the same color but the flash now looks blue. That because the camera white balance was set to adjust for the incandescent light which “cooled” the temperature of the entire scene. To get the flash the same color temperature I put a warming gel over the flash.
This last picture was the result. The light its self looks natural and the flash around the light looks natural as well.