What color is indicative of rust on metal?

Rust on metal is primarily characterized by a reddish-brown color. This color results from the oxidation of iron when it reacts with oxygen and moisture in the environment, forming iron oxide. The process typically occurs when iron is exposed to these elements over time, resulting in a corrosion product that is often referred to as rust. This reddish-brown coloration is commonly observed on rusted iron and steel, making it the definitive visual indicator of rusting. Other colors noted in the choices, such as green or blue, correspond to different forms of corrosion or patinas on other metals but do not signify rust specifically.