I contacted Larian and this is what they said:
The color of an item/weapon shows the powerfulness. The color range goes from weak to powerful:
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 the 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
Now we know!
