[green ], in heraldry, signifies love, joy, and abundance. It is represented on the shields of noblemen by the emerald, and on those of kings by the planet Venus
In heraldry vert is symbolically expressed by diagonal lines running from right to left of the shield. Lines running the reverse way—i.e. from left to right- mean purpure.
N.B. English heralds vary escutcheons by only seven colours, but foreign heralds employ nine colours. (See Heralds.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Vert from Infoplease: