Instead of editing your images in post to inject color into your shadows, you can use gels to do it in-camera. Here’s a 6-minute video from photographer Jay P. Morgan at The Slanted Lens demonstrating 4 different methods.
1. Foam Core Mount
The first method uses a white piece of foam core as a fill card with the gel taped on to it. This will act as a colored fill light, and bounces the color of the gel into the shadows.
2. Mirror Mount
By using a mirror in place of the foam core you can create a harder colored light on the side of the face for a more intense look.
3. Gel Used Directly on Strobe
This is a more common way of using gels. You can position the strobe to use it as either a rim light or a hair light and apply the gel directly to the strobe.
4. Gel Inside Softbox
This will have a broader effect across the entire image, not just on one side. Place the gel inside the soft box and then position it either above or below the camera, using it as a fill light. Morgan notes that you should dial back the light enough so that the color doesn’t create issues with the natural skin tones.