请翻译：美国人开放的作风

I rather translate 作风 as "attitude and lifestyles". Because when you say "作风" alone it is more about the general thinking, believing, and acting. Otherwise, you have 工作作风， 领导作风，等等。

So, I would translate the whole phrase as:
the liberal attitude and lifestyles of Americans

the liberal style of Americans

the exoteric style of American.

American exoteric style