A material presents transparency when it allows light to pass easily. Transparency is an optical property of matter, which has varying degrees and properties. A material is translucent when it lets in light, but it does not allow objects to be clearly seen. Instead, it is opaque when it prevents the passage of light.
Red is one of four primary psychological colors, along with yellow, green and blue. In addition, it is considered a warm color, along with orange, yellow and all the colorations that tend to these. In nature, red plays an important role in the warning coloration, also called apostematic, of animals to warn predators of their toxicity or bad taste.