Why does "The Open Boat" start off with the line "None of them knew the color of the sky"?

Stephen Crane's "The Open Boat" begins with the following passage:

None of them knew the color of the sky. Their eyes glanced level, and were fastened upon the waves that swept toward them. These waves were of the hue of slate, save for the tops, which were of foaming white, and all of the men knew the colors of the sea. The horizon narrowed and widened, and dipped and rose, and at all times its edge was jagged with waves that seemed thrust up in points like rocks.

The story relates the tale of Crane's real-life shipwreck in 1897. While traveling from Florida to Cuba, the ship he was traveling on struck a sandbar and sank. Crane and three other men were stranded at sea for 30 hours. One of the men in the lifeboat perished when the craft capsized.

So, reading the passage, we see a grim group of terrified men, their eyes glued to the surrounding water, which is their immediate danger. They are so fixated on the sea and watching for danger or waves that could upset their craft--and possibly for ships that could rescue them--that they have no attention to spare for any details but those of the sea, not even for a glance at the color of the sky. Note how the men are intimately acquainted with the color of the water, however, and with the appearance of the horizon. It is the only thing that concerns them, because it is their immediate source of danger.

The story opens with this passage to emphasize the mental state of the men in the lifeboat, their grim fixation on the surrounding sea, their fear, and also their sense of being lost and isolated out in the open water.