He who is parakeet wherever he wants is green" is a popular saying that means that he who has an ability should not have difficulties when a demonstration of it is required by circumstances.

It applies to those situations in which a person, who stands out for having certain qualities or talents, must demonstrate them, regardless of the situation or circumstances.

This saying is also a kind of warning: you should not brag about what you do not have or know, because sooner or later, you will be exposed.