As seen on the last page of the book, it's seen that Nick is observing to the fact that Gastsby "had come a long way to this blue lawn..." Seeing as how Gatsby is connected to this "blue lawn". You can connect the blue lawn back to Gatsby and his house which can connect to the parties in his home. (pg. 39 - a majority, if not, all of chapter 3) the parties in Gatsby's house were not real in the sense that it wasn't just a normal party. People go to Gatsby's house to escape from their realities and just be "out of this world" for just a little while. blue in association with Gatsby signifies Gatsby's ability to transform the ordinary and give it a deep romantic appeal, a freshness and a newness.
And also with the color blue, it can also mean sadness or sorrow.
Tom's car is blue. Maybe the blue car represents the sadness and unhappiness in Tom and Daisy's marriage with it being based on money rather than love. With blue meaning both fantasy and sadness we can connect with the blue car that in the back of his mind, Tom is driving towards a future in which him and daisy could be connected by love and not money. but knowing what his present is, he knows that that dream is only a fantasy. but he keeps on driving, in hopes that one day, this fantasy could become reality.