What term describes a light filter that changes the hue of a beam?

The term that refers to a light filter that changes the hue of a beam is "gel." Gels are made from thin sheets of colored material that can be placed in front of stage lights to alter the color of the light they emit. This is essential in theatrical lighting design, as it allows the designer to create different moods, enhance the atmosphere, or signify changes in time or location through color manipulation.

The other options focus on different aspects of lighting: focusing pertains to the adjustment of the beam's sharpness and spread, fade up/down describes a change in intensity rather than hue, and a profile spot is a type of lighting fixture equipped with a lens for creating sharp beams and patterns of light, but it does not inherently change color without a gel. Therefore, "gel" is specifically the correct term that directly addresses the alteration of light hue.