First of all, keep in mind that symbolism is when an object in the story represents a deeper meaning. This means the a symbol must be an actual object in the story first. Characters and colors can also certainly be symbolic.
I would first start by taking a look at the third link below, and reading a little bit about traditional color symbolism for many different cultures. Starting here might allow you to think back to the text and realize how some predominant color description may or may not be linked to a traditional symbolic meaning. Start a list of ideas, even if you are not sure that you are correct at this point.
Then, go back to the text and continue your list of possible symbols throughout the story. Often, the meaning of symbols is exactly as obvious as you would expect it to be. Look at characters with distinct personalities. What about this character (objects or colors often associated with him or her) could possibly be symbolic and how does it relate to the character's personality or attitude?
Once you have compiled a list of ideas, go back to the text and see if you can find concrete evidence to support them. Where ever possible, jot down an example, either in the form of a quote or paraphrased explanation. Then, organize your essay like you would any other essay. Group similar ideas, come up with a thesis statement, and outline.
If you find that you are struggling beyond the brainstorming task, feel free to post again on Enotes with specific questions or concerns.
Thank you very much,I will follow your advice and will try