When it first appeared in the language, virescent was a poetic way of describing a greenish hue (it was taken directly from the Latin verb virescere, to become green, itself from virere, to be green).
Past the creamy reef the purple ocean glittered in the nooning sun, while the motionless waters of the lagoon were turquoise and bice near by and virescent in the distance.
Mystic Isles of the South Seas, by Fredrick O’Brien, 1921. Bice is a dated term for a medium blue or blue-green copper-based pigment. Like so many colour words its hue has changed over time — when it came into English seven centuries ago, it meant a dark or brownish grey, from Old French bis, dark grey.
The adjective has been used for a deathly hue:
Between them stood a table covered with green baize, which, reflecting upwards a band of sunlight shining across the chamber, flung upon his already white features the virescent hues of death.
The Hand of Ethelberta, by Thomas Hardy, 1876.
Virescent has since changed its sense, in particular among plant scientists. It refers to the normal green of chlorophyll that has been shifted towards yellow for some reason, often because of disease. Virescent mutants lack chlorophyll in their young leaves, which look yellowish in consequence. Conversely and confusingly, the related noun virescence can mean an abnormal development of a green colour in parts of a plant that normally aren’t green.
A few writers have used virescent where others would prefer verdant, meaning the rich green of flourishing plant life, which derives from a related Latin word:
County Kilkenny is a beautiful grassy wonderland of virescent pastures, purling waterways, winding roads and mossy stone walls.
Ireland, by Fionn Davenport et al, 2006.