We hate to think about the amount of ink that's been spilled writing about the green light in Gatsby. This is a grade-A, prime-cut symbol: the "single green light" on Daisy's dock that Gatsby gazes wistfully at from his own house across the water represents the "unattainable dream," the "dream [that] must have seemed so close that he could hardly fail to grasp it" (1.152, 9.149).
Okay, you're right: it's not quite that simple. The green light also represents the hazy future, the future that is forever elusive, as Nick claims in the last page of the novel: "Gatsby believed in the green light, the orgastic future that year by year recedes before us. It eluded us then, but that's no matter – to-morrow we will run farther, stretch out our arms farther…." (9.149). But if the green light represents Gatsby's dream of Daisy, in the past, then how does it represent the future, as well? Is the future always tied to our dreams of the past?
One last thing. Red-green traffic lights began to be installed in the U.S. in the 1910s and 1920s. Coincidence? Maybe. But our money's on not.