The houses are haunted (1)
Ghosts aren't real, but they do exist in our imaginations—and movies, of course. But the figurative language has already got our creative juices flowing. Our minds can turn these normal suburban houses into haunted ones. Neat.
By white night-gowns. (2)
White nightgowns represent our base level of imagination—meaning this is what a department store sells us for cheap. Sure, one may have a lace collar, and another might have ¾ sleeves, but for the most part, they all look just about the same.
None are green, (3)
Wearing a green nightgown takes a little bit of imagination and a little more money. You'd have to find a cool store to buy it in, but you could still buy it. And at least you'd be snazzier than everybody else. That's worth a few bucks, right?