Items in Dying Light are ranked by their "Rarity". The rarity are ranked by different colors White, Green, Blue, Purple, Orange, Gold.
White (Common): White items are extremely common and are more used for items rather than weapons. The only weapons with the White rarity are pipes, planks, and shovels. Basically any weapon found in the open.
Green (Uncommon): Green are still extremely easy to find but not as common as White. Green rarity weapons are mainly found on enemies such as Rais's Men.
Blue (Rare): Blue weapons are mainly found in medium lock picks. Some items are even blue rarity being Power Cables
Purple (Legendary): These weapons can be found in hard lock chests. A Electronic is a purple item that can be randomly found.
Orange (Epic): Orange weapons can be found in hard locks or very hard locks. Bolter Tissue is a orange item that can be found after searching a recently killed Bolter.
Gold (Nearly impossible to find): Gold weapons can be found in hard or very hard locks if playing on Hard or Nightmare difficulty at night. They can also be earned in "Be The Zombie" mode. To earn them you must be ranked as "Underdog" as survivor. There are no known items of gold rarity.