Definition - What does Green mean?
Green is a tasting term commonly used to describe wines that are made with unripe grapes or are very young, and thus high in acidity. It is a tasting term that denotes the presence of vegetal and herbaceous characteristics in both the nose and flavors of a wine. These characteristics are akin to grass, leaves and green vegetables (green bell peppers, asparagus, etc).
Green wines can be young and acidic, made from unripe or under-ripe grapes or made from must that has been left in contact with the stems and seeds for too long.
WineFrog explains Green
Green is commonly used to describe wines made from unripe or under-ripe grapes. Grapes like Cabernet Franc, Merlot and Malbec produce wines that have stems, leaves, vegetables and other green flavors instead of fruit flavors when made with unripe grapes. Also, the smell and flavor tend to remind most wine critics of mowed lawn or grass. Other common terms applied to these wines are grassy, herbaceous, vegetal, stalky or stemmy. All of these terms have nuances that differ from the general term of “green".
Another application of the term “green" is to denote wines that are young and developing. These wines will feel harsh and astringent on the palate, due to their high acidity, and will usually remind wine critics of chewing a stalk of grass or tea leaves.
However, green does have a positive side – just like not all young wines are bad. Certain varieties, such as Riesling and Gewürztraminer, benefit from green characteristics. In small doses, Sauvignon Blanc will also benefit, as it adds depth and character to these lighter wines. When the green flavor is followed by gooseberry or apple flavors in a white, it means the wine will be refreshing and zesty. In reds, especially those grown in cooler climates, a touch of green leaves gives an interesting depth and mouthfeel (tartness).
The key to deciding if the green characteristics in a wine are beneficial or not are the power of these characteristics. If the smell and taste of green overpowers all other flavors, then the wine is bad. If the smell and taste of green is an undercurrent, complemented by fruit, acidity, tannin and depth, then these characteristics are beneficial and the wine is perfect for your evening sipper.