It helps me to think of "the curtains are blue" style symbology like this: The entire world of the story is constructed. If the author wants the character in a house, wants to describe the house so that the reader can picture it, they must build it from the ground up. They must then choose which aspects of the house to highlight in their description, knowing that the details they mention will draw the reader's eye.

Not every author makes everything symbolic, but some author's use this deep control of the world and what the reader notices to further their point. The curtains may be purposefully blue not bc the author sat there thinking blue = sad but because they want to place cool tones in the house, the way an artist might to bring the mood of a painting down.

Curtains are blue is less about literal one to one symbolism and more about using setting, lighting, and, in your catcher in the rye example, blocking, as tools to further atmosphere