What is a piece of colored plastic that blocks the passage of light except for its own color called?

The term "color filter" refers to a piece of colored plastic designed to absorb certain wavelengths of light while allowing others to pass through. This property makes it essential in various applications, such as photography, stage lighting, and visual arts. When used in lighting, a color filter only allows light of its respective color to pass through, effectively blocking all other colors. This selective transmission is critical for creating specific moods, enhancing visual effects, and achieving desired color mixing in performances and productions.

While other terms like “light gel” and “color gel” are often used interchangeably in the context of theatrical lighting, they typically refer to a specific type of color filter used with stage lights. In that sense, "color filter" serves as a broader, more encompassing term that describes the function and purpose of the material. "Color screen" is less commonly used in the context of lighting and may suggest a different application, hence it is less appropriate in this context.