John's not a terribly poetic man. He doesn't often give himself to flights of fancy. Even if he were, though, he doesn't think he could ever do Sherlock's eyes justice. When it comes to them, none of the metaphors that come to mind are ever sufficient.
The writer inside of John has compared them to moonstones, with their grey-blue iridescence. To a celadon tea pot, a jade hairpin they came across on a case, back in the early days. Sometimes he thinks of a mirror, which makes no sense. Mirrors have no colour of their own. The most frequent comparison though, and probably the most apt, is London herself. Blue sky, green water, grey smog, clouds and pearls and smoke and snow and a million impossible things.
Sherlock is staring out the window, his unwavering gaze locked on the skyline of the city. Those eyes, they're pale right now, somehow almost white. Pure and unblemished, the better to absorb everything he sees.
The consulting detective turns his back to the window and sees John there, as if aware of him for the first time that day. For a moment those eyes fixate on him and John freezes. They draw the eye contact out a fraction longer than friends, than flatmates, until pointedly, as if to fracture the sudden tension, Sherlock blinks.