If it's used as response as in 'It's correct' (i.e. Your statement is correct), this should be "правильно" (adverbial use). So this is a simple positive statement to express agreement: Правильно. (That's correct). 

But it can be an adjective as well,
as in 'the correct premise' -translates "правильное допущение", or 'correct answer' = правильный ответ.