The color of item shows how powerful it is. AFAIK the color range goes from weak to powerful like this:
White - Usually no bonuses
Gray - Usually one bonus
Blue - Usually 1-2 bonuses
Green - These can be good and can be found relatively early in game
Yellow - These are powerful items with probably some charm slots and a few bonuses
Orange - These are the most powerful ones. Usually quest items and uniques of that type.
