When you are shooting indoor without window or outdoor at night, but the existing light is not bright enough, you may use a gelled flash to match the existing light and create a documentary photograph.
But how to figure out what color to gel? Use the jpeg quality setting and set the white balance to daylight, make a correctly exposed frame of the existing light with no flash added – looking at the this image on the LCD will give you a starting point but if you can’t really tell if the image has a red, orange, yellow, green, yellow-green, purplish or peachy color cast.
For example, you are in a bar shooting an event, and this is what it looks like when your white balance is at daylight:
Because of the tungsten, the image is in the orange tone. So I chose to use a dark orange gel to gel my flash. This is how the image looks like after gelling my flash and setting my white balance to tungsten: