The green light is not introduced until the very end of the first chapter. However, Gatsby is also not heard from or seen until that same moment. Right away, the reader can deduce that the green light will be significant to Gatsby, as it appears to be the only reason Nick saw him at all.
Nick describes Gatsby as:
"content to be alone...he stretched his arms out toward the dark water...I could have sworn he was trembling" (p. 25).
These few sentences clue us into the emotional importance of the green light. We have already discovered that the rich people on the Eggs like to be seen and heard - as evidenced by Tom and Daisy - so there is significance in the fact that Gatsby appeared content to be by himself at that moment. Also, Nick said that Gatsby was trembling. People do not tremble for no reason; typically they tremble because they are cold or as an emotional response. As it was a warm summer night, one can rule out Gatsby being cold. Therefore, the trembling must have been brought on by some powerful, emotional response, perhaps sadness, longing, or excitement.
The rest of the chapter introduces the meanings and significance of color, and green can be seen as bright and powerful, the end of the chapter in which Gatsby is finally introduced provides the most textual evidence that green will signify strong emotion for him.