Here in Uruguay if you say "El pasto del vecino es mejor / está más verde", se entiende muy bien. I think that in Spain is " las manzanas siempre parecen mejores en el huerto del vecino". Does it help?
I'm not sure in what countries they use this saying however it's understood in Mexico at least. By the way they would say pasto here in Mexico. I think it's really cool when every once and a while translations like these work in other languages and the same feeling of the expression is conveyed in another language.