The bride in this Renaissance masterpiece wears green as a symbol of her fertility. She is slouching in imitation of pregnancy, thus indicating her willingness to bear children.
In Celtic myths the Green man was the God of fertility.
Later in the millennium, Early Christians banned green because it had been used in pagan ceremonies.
Nevertheless, as evidenced by this 15th Century wedding portrait, the color green was the best choice for the bride's gown because of its earliest symbolism.
Of note is the continued symbolism attached to the color in the latter part of this century. Anyone who chooses a green m & m (an American candy which contains an assortment of different colored chocolate sweets) is sending a somewhat similar message. Green has been reinterpreted by late 20th century American culture to signify a state of heightened sexuality in this specific situation.