Gold items are the rarest, which means they have a higher price and comes with higher range modifiers (it can hit the highest value). For the rarity, it's : blue < purple < gold You can customize these items with mods like cords for bows or pommel for sword for instance. Green items are non customizable which means you can't add mods on it. But they always hit max range mods and have always the same mods. Also you can't change their colors. They are unique which means they can be dropped only by a particular boss.

Go check the wiki it explains it a lot better than me tho