Green apple mostly functions as a plain fruit or health-food signal, but it's also become shorthand in some fandoms and slang circles for 'unripe' or 'sour' personality traits, playing off its tarter taste versus the red variety. Part of the original 2010 Unicode fruit batch, its coloring occasionally gets read as unripe or 'not ready yet' in figurative captions.
Keywords: apple, fruit, green