How is the light different before he is reunited with Daisy as compared to after he is reunited with Daisy?

Before he is reunited with Daisy, the light symbolizes hope for things to come. After they are reunited it loses its significance and is nothing more than a light at the end of the boat dock.

"If it wasn't for the mist we could see your home across the bay," said Gatsby. "You always have a green light that burns all night at the end of your dock." Daisy put her arm through his abruptly but he seemed absorbed in what he had just said. Possibly it had occurred to him that the colossal significance of that light had now vanished.