As dark and dreary as the tone of Across Five Aprils is, every once in a while Hunt throws in a rainbow of color as if Dorothy just opened the door into Oz. Since the atmosphere of the Civil War is pretty depressing, any pop of color or light is not only wholeheartedly welcomed but also bound to have significance.
Whenever we see color (or lack thereof) it's in parallel to the circumstances or emotions of a character. For instance, when Jethro goes to visit Shad it "changed the color of the world" (4.39) for him, meaning that all the doom and gloom melts away through the power of Shad's awesomeness. But on the flip side, Jenny's face being "drained of some of its rich color" (6.143) is essentially the life getting sucked out of her from all the anxiety she feels as the Creightons repeatedly get harassed.
Hunt also uses color in motion to mirror feelings and mood. When expected to read his VIP letter from the President out loud to this family, everyone around Jethro "spun in a strange mist of color" (9.162). No, this book hasn't suddenly leapt across genres and decided to hang out in fantasy land—Jethro is just super nervous about coming clean about the letter, so he isn't focusing on one person or thought. Hence the mist.
But nothing is more exciting or vibrant as when the final April of the book arrives with "a burst of warmth and color" (12.57). This "burst of warmth and color" lets us know that something has shifted in a major way and for the better—and in this case, it means the war is finally over. Now the nasty darkness that's hung over the previous four Aprils can go away.
The moral of the story is this: in Across Five Aprils, color consistently clues us into how characters are feeling.