Once in a (red/ blue/ yellow/ purple) moon
Once in a blue moon: very rarely, occasionally
(red/ green/ blue/ purple) with envy
Green with envy: jealous
A: "Sometimes, I think that I'll be happier teaching in Spain." B: "Well, (the grass is always always greener on the other side). You have already changed your career four times."
other people's lives or situations always seem better than your own