1.chase rainbows
to try to get or achieve something that is difficult or impossible

2.give (someone) the green light
– to give someone permission to proceed with a project

3.green-eyed monster
– jealousy

4.greenhorn
– an untrained or inexperienced or naive person

5.red-letter day
– a day that is memorable because of some important event

6.show one’s true colors
– to show what one is really like or really thinking