Green and Yellow
Red is out (for 2 reasons)
1) The human eye has a lower sensitivity to red than green or yellow. This causes the red fiber to fade out faster as dark approaches.
2) As a glasses wearer, when looking through my glasses (high index lenses) at the red pin, in order to get it to "appear" in line with the rest of my pins, it actually has to be about 1/32" to the left when looking straight on, however at full draw it looks perfectly in line. This has to do with the lens in my glasses acting like a prism and causing the red light to defract at a different angle than the green or yellow.