Green is everywhere in Esperantujo. It is rare to find an Esperanto-related website or item that does not contain that color.
The origin of the use of green can be traced back to Dr. Zamenhof. In 1911, he wrote:
About the origin of our green star I no longer remember very well. It seems to me that Mr Geoghegan drew my attention to the colour green, and from that time I began publishing my works with a green cover. About one brochure, which I quite by chance published with a green cover, he remarked to me that this was the colour of his homeland, Ireland. Then it came into my head that we could well regard this colour as a symbol of Hope. As for the five-pointed star, it seems to me that Mr de Beaufront first printed it on his grammar-book. I liked this and I took it as an emblem. Afterwards by association of ideas the star appeared with the colour green. (source)
So Zamenhof was influenced by Richard Geoghegan, an Englishman of Irish descent, and he thought the color green might be associated with hope.
The house where Zamenhof was born had the address Zielona 6 (source (in Esperanto)) in Poland. Zielona means ‘green’ in Polish, hence this might also be why green is associated with Esperanto.