푸르다 is used to describe the forest or wood. for example, 저 산은 정말 푸르다.(That mountain is so green.) 파랗다 is used to describe the sea or the sky. for example, 저 바다(저 하늘)은 정말 파랗다.(That sea is so blue.) This is my first time to correct something. please understand if my correction is bad. :)