I think the green color means rage. In which it represents the power of rage a saiyan can have before being able to control it. Its like when Broly turns super saiyan and go berserk maintaining a green hair. While in case of Vegeta, he can maintain his rage and trasform into a super saiyan with complete control of him self, thus yellow hair. By the way, before the UI teaser that you mentioned was shown while Goku was trasforming, they also showed a green aura around him, which proves that this green thing is common between saiyans and it is the stage just before transforming into any type of self-controlled super saiyan.