That's actually a great question if you're not familiar with the TFX Pro sights. They are both Fiber Optic and Tritium night sights. There's an orange ring around the front sight to help you acquire and place the front on target quicker.
In darkness, you see only the green tritium glow front and rear.
In medium light, you see the green rear sights getting brighter from the addition of fiber optic light; and also the front sight is starting to get brighter and you begin to see the orange ring around the center point.
In bright ambient light, the front fiber optic really kicks in and glows brightly, ditto for rear.
I hope this info helps.