A colored slide is a type of slide used in a slide projector, which is a device used to project an image onto a wall or screen. The term 'colored slide' refers to the fact that the slide contains a color image, as opposed to a black and white image. The term 'GEL' is a three-letter abbreviation for gelatin, which is a material used to make slides. Gelatin slides are often used in scientific experiments, such as microscopy, where they are used to view specimens under a microscope.