It was said that Tom drove a blue car, and Gatsby drove a yellow car.
This can be seen in the movie, and is also described by Fitzgerald, that wonderful master of the English language.
The green light as seen at the end of Daisy’s dock can be interpreted to represent Daisy herself.
So, is it a coincidence that the cars were blue and yellow? Two influences of Daisy, Tom and Gatsby, each have an impact on this confused woman, and the colors of their cars combine to produce the color green.
I assume Fitzgerald did this on purpose, because I wouldn’t put something as wonderfully minuscule such as this over his head.