Do students colour the stars, do you give them stickers?
i put a stamp &/or sticker on the star. once they got 10/20/30 stickers or stamps, they'd get a reward (ie. pencil, notebook, candy...).
i wouldn't color the stars as students could color stars at home and next thing you know, everyone has 30 stars filled up. also, coloring stars in class might take up too much class time.
hope that helps