It may surprise you to learn that green tea and black tea originate from the same exact plant species, the Camellia sinensis. Essentially it is the variety of tea plants and how the tea leaves are processed that define how green tea becomes green and black tea becomes black.
Typically a brewed green tea is either green, yellow, or light brown in color, and its flavor ranges from grass-like and toasted to vegetal, sweet, and seaweed-like. If brewed correctly, most green teas are quite light in color and only mildly astringent.