Two scenes from The Year Without a Santa Claus—both show Santa in a neutral or cool-colored environment. The bedroom is gray; the snowy night is gray and blue. Close by Santa, however, is a warm orangey-yellow light source.
This is an old trick. If you look at classic Nativity paintings (you may have one handy on a Christmas card), the artist will keep everything in the barn dark and/or neutral while the manger throws off warm light, illuminating Mary and Joseph. The viewer will be attracted to the light source, where the artist wants you to look.
It’s not easy for children’s book illustrators to make backgrounds as dark as we would like, because often text needs to print over the background.