What does it mean, "The curtains are blue"?
Since I've read one too many things on the curtains recently, I'll let you in on the secret:
The story will tell you.
(And your English teacher should have told you that.)
Sometimes it means that the writer was sloppy and wrote superfluously, but generally not. There's generally a reason, however trivial, why the curtains are blue rather than red-checked, or floral prints, or non-existent (in story), or omitted (in the description).
It could be that the writer is going for a maximalist style, full of details and filigree, but unlikely, because "Sky-blue curtains fluttered" or "Royal-blue curtains hung, weighted" pack in more details.
There are two things to consider up front.
One is the observation of J.R.R. Tolkien:
I think that many confuse applicability with allegory, but the one resides in the freedom of the reader, and the other in the purposed domination of the author.
A story, or an element of the story, can have meaning for you without its being the story's meaning. This is not a easy topic for some people to learn nowadays. I have read people who insisted that every single story was about their current-day fad, and their standards of evidence were such that they could not argue against the claim it was a solar myth.1
Another is the words of Ernest Hemingway
Then there is the other secret. There isn't any symbolysm [sic]. The sea is the sea. The old man is an old man. The boy is a boy and the fish is a fish. The shark are all sharks no better and no worse.
In more abstract and thus general terms: anything in the story that serves an actual purpose toward plot, character, or setting has that purpose in the story. If it is symbolic, that's on top of that purpose, and you need actual evidence that it's symbolic.
And there are many, many, many purposes that blue curtains can serve in the story.
Consider these passages:
"You can't miss it. The curtains are blue."
And, indeed, among all the cottages with innocuously white curtains, one sported blue curtains.
vs.
"You can't miss it. The curtains are blue."
And indeed, among the motley of purple, pink, green, white, orange, yellow, purple and yellow striped, green with pink flowers, and white with orange dots, there was only one set of blue curtains.
They walked around the cottages twice to be sure of it.
Both of them characterize, but the first one tells us more about the person in the cottage, and the second about the direction-giving person.
Or consider
The dimension machine landed on the rocky summit, which was indeed a rocky summit. Pines surrounded it, and the fallen needles were amber in color and scented the air. He hefted his pack and walked down the trail, counting off each landmark. The forest opened to a clearing where a stout cabin stood. It was built as a log cabin -- check -- it had a porch extending the length of it -- check -- it had --
The curtains were blue.
His shoulders slumped.
Mind you, this only proves that he's dejected.
It could mean that he once again landed in the wrong dimension.
Or it could mean that while he hopped dimensions, life has gone on at home, and the curtains are a sign of how he will have to adapt to changes.
In those cases, that the curtains are blue is incidental. (Though more context might change that.)
Then, given this description
The park was filled with flower beds. There were roses and lilies and blue delphiniums. Past that was the hall. Three receptionists spoke with those who visited. One wore a blue dress. Another one gave directions to the office. They went down four corridors, with carpets that muffled footsteps, and the third corridor's carpet was blue.
In the office there were seven men in suits. One suit was blue. There was a big desk. Three windows were covered in curtains. The curtains were blue.
the reader will presume that the color blue meant something to the viewpoint character. Probably something crazy since he's distinctly nuts.
But in this context,
Seven years and a day after the Queen died, the commons and lords presented a petition to the King. He did have the black curtains and banners taken down, but only to replace them with blue. His own clothes ceased to be black, but turned only to blue, or purple, or gray. He donned no jewelry.
Courtiers and commoners alike grumbled that a king with no heir should doff even second mourning before his queen was dead three years.
there's characterization, but also setting. Plus a problem likely to at least influence the plot.
For more setting, perhaps
They did not hang them with glass eye beads to ward against the evil eye, but still they hung blue curtains.
which even hints at the magic of the land, or
The curtains were blue, would you believe that? All about, people hung up good frugal white, and these people bedecked, not only themselves, but their house, with blue. How extravagant.
which talks of economics.
Then again, you could have a corridor.
Periodically, along the long white corridor, sky-blue curtains hung, completely blocking the windows, shutting out any hint of the outside, any sight of the sky, and any hint of the time.
where "sky" is doing a lot of the lifting. "Midnight blue" would also work. Just a touch of irony that the viewpoint character can't in fact see whether the sky is blue, or the hour midnight.
Just a touch to be sure. Other details are needed to reinforce the image. On the other, it's only two words. There's only so much lifting it can do.
Even in a passage such as,
They walked into the castle from the dark night, shedding their cloaks. In an ante-chamber of white marble, sky-blue curtains covered the windows, and in the middle, the princess, her blond hair falling over a gown of the same yellow, her smiling face radiant, awaited them.
the obvious hint of solar symbolism for the princess will need reinforcement if she is to be thoroughly connected to the sun.
Such is the art of writing.
In the nineteenth century, a vast school of academics interpreted all sorts of stories as solar myths. Hansel and Gretel was a solar myth. Sleeping Beauty was a solar myth, and The Frog King -- and Napoleon Bonaparte, by the standards they used. Learning about it is an excellent way to inoculate yourself against easy assumption of meanings.