Why does Margot dream of a gold or yellow crayon?

This question refers to the story "All Summer in a Day" by Ray Bradbury.

In this story, we meet a group of children who live in the planet Venus, where the sun only shines for two hours every seven years. Most of the children live relatively happy, but Margot finds it unbearable. She used to live on Earth before, and the lack of sun has made her sickly and extremely unhappy. She dreams of a gold or yellow crayon because she associates these with the thought of the sun, which is all-important to her.