The color green is used in World of Warcraft in various different ways. One way is as part of the usual Blizzard quality color system for items (gray, white, green, blue, purple, orange, gold) or difficulty for mobs (gray, green, yellow, orange, red) and tradeskill recipes (gray, green, yellow, orange).
Some items' names appear in green letters. This is part of the item quality system: "green items" (whether Weapons, Armor, Trinkets or something else) are of Uncommon quality, better than Common but not as good as Rare.
Some mobs' level numbers appear in green digits, although the color depends on the level of the character who views them. "Green mobs" are mobs that, despite being of lower level than your character and therefore relatively easy to kill, still yield experience. Gray mobs are too low level for you to get experience from them. Yellow, orange and red mobs are more of a challenge and therefore worth more experience to you.
Even though a level 80 character can no longer accumulate experience, the difference between a gray and green mob is still important to level 80 Warlocks: they can only obtain a Soul Shard from a green or better mob.
If the overhead name of a player, mob or NPC is green to you, this signifies that the unit in question is friendly to you but attackable by enemy players (their PvP Flag is on). Flagged players who are in your party have light green names; otherwise, flagged players' names appear a darker green.
Green in Professions
In your profession window, a recipe shown in green will rarely give you a skillup when used. These recipes are considered "moderately easy". The same is the case for the gathering professions, a dead body for example, which shows "skinnable" in green will only have a small chance of giving you a skill point, whereas a yellow one will have a high chance and an orange one always will. A gray one means that it's below your skill level and will never give you a skillup.