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.