Color gels are mainly used for two things:

1. Creative color effects — putting a colored gel on a flash or light to make part of the scene red, green, blue, etc.
2. Color matching — adjusting a flash to better match ambient light so the scene is lit by a more consistent color temperature.

The key advantage is that gels change the light before it hits the subject. In scenes with multiple light sources, each light can affect different parts of the image differently, and those contributions often overlap. Once the photo is taken, you usually can’t reliably separate which light caused which color in every area, so post-processing can’t fully recreate or fix that.

If all you want is a global color shift across the whole image, post-processing may be enough. But if you need one light to differ from another—or one light to match the others—gels are the practical solution at capture time.

They also let you judge the mood and balance on set and keep that look consistent across different compositions and reshoots.