What are plastic sheets of specific colors placed over lighting instruments called?

The correct term for plastic sheets of specific colors placed over lighting instruments is gels. Gels are utilized in film and theater production to change the color of the light emitted from a lighting instrument, allowing for desired visual effects and mood enhancement. They come in a variety of colors and can also manage temperature balance, helping to set the appropriate tone for a scene.

Gels are designed to withstand heat and are critical for controlling the appearance of lighting in a production. They support the creative expression of the cinematographer or lighting technician by providing the ability to easily adjust the light's color without the need for complex alterations to the lighting setup.

Filters could refer to any device that alters a physical property of light, but they do not specifically refer to color gels. Diffusers are used to soften or spread out light rather than change its color. Modifiers is a broader term that could encompass a variety of tools used in lighting but does not specifically identify the colored sheets used over lights.