Why is Saturday "battleship grey"?

The author's depiction of Saturday as "battleship gray" is highly symbolic. On a literal level, it is late winter, and the sky is, in fact, a deep, dull, overpowering gray color. Symbolically, however, the color of the sky represents the ominous reality of the war which overshadows everything that winter of 1943.

Earlier in the chapter, the winter sky is described as "an empty hopeless gray," which in the context of the story reflects the inexorable encroachment of the war on the lives of the youths at Devon. The halcyon days of summer are over, and although Leper is the only one so far to take the initiative to actually join the armed forces, it will not be long until the others will be swept along with the tide as well. As the author states, "only Phineas fail(s) to see what (is) so depressing;" doomed by his injury to be left behind when his comrades will inevitably be taken by the military machine, he creates his own world of carefree innocence, the titular "separate peace." For one day, the boys are swept into Phineas' version of reality, as they are seduced into participating in his "Winter Carnival," his "choreography of peace." Despite the merriment Phineas creates, however, the sky remains "battleship gray;" the oppressive presence of the war will not go away. The reality of the situation is brought home abruptly at the end of the chapter, when Gene receives an urgent telegram from Leper. Its contents jolt the boys back to the real world, and its stormy, threatening conditions which cannot be denied (Chapter 9).