"The purple bird sings."
Translation:보라색인 새가 노래합니다.
putting 'the' in every nouns wouldn't be so natural; Korean grammar is very different to european grammar so a natural sentence in Korean might not sound so natural in English, but you must leave it that way. This is because '보라색인 새' could mean a specific bird but it could imply the meaning that 'if a bird is purple, it sings.' The idea of trying to translate Korean phrases in one type of English phrase is misguided, as I say as native Korean myself.
Descriptive verb: 보라색-이다 = be of purple color = be purple.
Adjective form (present tense): 보라색-이 + -ㄴ = 보라색-인 = that is purple
보라색인 새 = bird that is purple = purple bird.
Note: "-색이다" (be of color) is used to describe all colors.
Neutral colors (black and white) and primary colors (blue, red and yellow) have their own sets of verbs, hence adjectives.
까맣다 = be black 까만 = black
하얗다 = be white 하얀 = white
파랗다 = be blue 파란 = blue
노랗다 = be yellow 노란 = yellow
빨갛다 = be red 빨간 = red