The color orange 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).
Orange Items
Some items' names appear in orange letters. This is part of the item quality system: "orange items" (whether Weapons, Armor, Trinkets or something else) are of Legendary quality, better than Epic but not as good as Artifact. These items are extremely rare and generally only come from a chain of quests or construction of epic or better ingredients.
Orange Mobs
Some mobs' level numbers appear in orange digits, although the color depends on the level of the character who views them. "Orange mobs" are mobs that are usually higher level than your character and therefore difficult to kill and should yield good experience. Yellow mobs usually around the same level as you and give decent experience. red mobs are very challenging and therefore worth more experience to you.
Orange Names
The overhead name of a mob or NPC appears orange to you if you have a reputation standing of Unfriendly with his faction. He doesn't attack you on sight but you can't talk to him.
Orange Recipes
In your profession window, a recipe shown in orange should always give you a skillup when used (assuming you have not hit the ceiling of your skill range). These recipes are considered "difficult".