Okay, since it's an adjective, the following sentence (see below), Is an adjective modifying "You"? Since it's an adjective, it must modify a noun, and there is only one type of noune, pronoun of You.

"You should be quiet"

Thanks in advance!!

We also say:
"You are quiet today. Everything OK?"

Taking the section, 'You ARE quiet':

'quiet' as a noun means:
(a) silence, calm :"The early morning quiet"
(b) free of disturbance or interruption BY OTHERS

So, with 'quiet' as a noun, your sentence would mean:
“You should be silence.”

Do you think that a metaphor is the intended meaning, rather telling someone not to disturb others by talking or making noise?

Take another example:
‘happy’ is the adjective, and ‘happiness’ is the noun.

“They are happy to see me.”
“You should be happy that you still have your health.”