He remembered that grey eyes were the keenest. Explain.

As Peyton
Farquhar is being shot at, he supposedly sees the shooter's eye "gazing into his own
through the sights of the rifle," which would actually have been quite remarkable,
considering the distance between himself and the shooter; the reader should be awared that this
observation is probably only a part of the fantasy that takes place as Farquhar dies. 
Apparently, Farquhar has previously come across some piece of literature that suggests that grey
eyes are superior to all other colors in regard to seeing (exceptionally) well at a distance,
which is a sign of a superior marksman.  Whether this notion is based on the rarity of the eye
color, its assumed relation to the eye color of animals known for their keen senses (wolf, hawk,
etc.), or some trait inherent to the color, such as a predisposition to be unbothered by the
glare of the sun, etc., is unknown.  At any rate, the color of the shooter's eyes and his
presumed prowess as a marksman definitely tie into the general idea of Farquhar's fantasy, in
which he appears to be a great, and almost superhuman, hero.