Tritium: one of the rarest nature resources on Earth. Now you can own a small amount (made artifically in nuclear reactions) by picking up the Guns Modifty Tritium Sight for your Marui PX4.
Although Tritium illumination won't last forever, when it comes to existence it will almost last for 12 years, long enough for you to aim your target in the dark.
Tritium will come with different colors but most of the time you will find green used on sight becasue our eyes just see green better. If you don't want to lose your target during night skirmish, just pick one up......what's more you can even find your gun a bit more easily in the dark.