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?
Or purple with green rings, Or green with yellow rings, Or yellow with blue rings. (4-6)
You would never find anything like these in a store. You would have to tie-dye them and do some real imaginative work. What's interesting here, though, is that as imaginative as these nightgowns are, they're also kind of a logical progression. The purple one has green rings, then the green one has yellow rings, then the yellow one has blue rings. What about turquoise polka dots? Or magenta flowers?
None of them are strange, (7)
Strange is the rare version of reality that we have to seek out. These people haven't made the effort to be strange. They're too conformist. And what a shame it is.
Only, here and there, an old sailor, Drunk and asleep in his boots, (12-13)
This is the only literal and concrete bit of the poem. The rest describes possibilities and what things are not. But these sailors, they're good enough as they are, because they're too busy creating an awesome dream world in their dreamy little heads.