Well Ana, unlike what rubis said, actually "brilliant" is also an english word with the same meaning in both languages.
So you would simply use it as it is.
"You are brilliant" = "Vous êtes brillant" (When speaking to a non-friend persons, even if he/she is young)
"You are brilliant" = "Tu est brilliant" (When speaking to a close friend, even if he/she is older)
The difference between "Vous" and "Tu" usage in French depends on the relationship you have with the person in front of you.
Formal/respectful (use "Vous") or informal/friendly (use "Tu").
Also when you speak to many persons, of course you should use "Vous".
Cheers