you de worthless greens and blues into dusts, essences, and shards.
the enchantrix addon coupled with auctioneer is a powerful combo. when you mouse over an item, you'll get a tooltip that lists the vendor price, the median price at the auction house, all the various things the item can de into, and the ah value of those dusts, essences, and shards. makes it really easy to decide whether to ah it, de it, or vendor it.