Yellow
From WoWWiki
The color yellow 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).
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Yellow Items
No items' names should appear in yellow letters.
Yellow Mobs
Some mobs' level numbers appear in yellow digits, although the color depends on the level of the character who views them. "Yellow mobs" are mobs that are around the same level as your character and therefore moderately difficult to kill and should yield decent experience. Green mobs are usually lower level than you and give slightly less experience. Orange and red mobs are more of a challenge and therefore worth more experience to you. (However, they are likely highly resistant to your spells and will take little damage from you!)
Yellow Names
If the overhead name of a player, mob or NPC is yellow to you, this signifies that the unit in question is unfriendly to you and attackable by enemy players (their PvP Flag is on), but you do not currently have your PvP Flag on. When you engage in combat with them, their names will turn red and your PvP Flag will be turned on.
Yellow Recipes
In your profession window, a recipe shown in yellow will often give you a skillup when used. These recipes are considered "moderately difficult".
