http://www.forcematerial.com/home/2016/10/23/the-real-reason-luke-skywalker-has-a-green-lightsaber
It’s clear, then, that there is a deeper meaning to the lightsaber being a new colour, even if the particular colour was chosen for a purely technical reason.
Luke’s lightsaber was green because it popped against the blue sky — and because, by visually differentiating his new lightsaber from his old one, the filmmakers were able to show us that Luke had become his own man."
Now Luke’s lightsaber’s green 'cause it’s one of just two* colours Jedi cultists get to have, and Luke’s a stickler for tradition.
*Unless you’re an inconsequential nobody like Mace Windu. Then you get to have your own unique colour.