In "The Yellow Wallpaper" by Charlotte Perkins Gilman symbolism permeates the atmosphere, setting, and even the mood of the main character, mainly due to the fact that the woman has no other option but to transfer her disparate thoughts and emotions onto objects. This is, perhaps, the only way that she can make sense of her current situation.
It is arguable that the first symbol that we see is the isolated estate to which she is taken. A big house separated from the rest of civilization, basically, is symbolic of how her own issue, as big as it is, has been just removed to a separate place- but has not been resolved.
The central symbol, which is the yellow wallpaper, is described under a very negative light
it is dull enough to confuse the eye in following, pronounced enough to constantly irritate and provoke study, and when you follow the lame uncertain curves for a little distance they suddenly commit suicide--plunge off at outrageous angles, destroy themselves in unheard of contradictions.
Here we already see the first signs of how the woman personifies the patterns on the paper in a way that reflects her own state of mind. Words such as "uncertain", "destroy", "suicide", "contradictions", are present in her subconscious, and the paper is slowly leading her to open up to her true emotions.
Then, there is the color. Although the actual color yellow may or may not have a specific meaning, we could argue that in different types of literature it has meant different things. We could say that it is the color of cowardice, or the color of the "cheap" (as in the 1890's coined term "yellow press", or "dime a dozen"). One thing is for sure: the paper is ugly. It makes her feel oppressed that this is what is important about it.
The color is repellent, almost revolting; a smouldering unclean yellow, strangely faded by the slow-turning sunlight.It is a dull yet lurid orange in some places, a sickly sulphur tint in others.
Here we see more words: sulphur (related to the smell of evil), "revolting", "unclean" and, most importantly, "dull". Such are her emotions regarding the paper which, again, is nothing but a transference of emotions from her mind onto objects.
The slow progression of her depression will end up in her tearing up the yellow wallpaper in order to liberate the woman who she believes is trapped behind. Obviously, this is another clear reference to her own situation, where she has been removed from a comfort zone and placed in what is nothing short of an experimental room; all in aims to calm her nerves after giving birth. Hence, the paper is the biggest symbolism in the story because it literally mirrors her state of mind.