These are the three types of butterflies you need to kill (at least one from each type) to unlock this achievement:
- Red (the most common)
- Purple/Pink
- Yellow
I've listed all the butterflies up to Chapter 7, where you encounter your first yellow butterfly. There are more in the game but they are not necessary for the achievement.
0:00 Intro
0:10 Red Chapter 2
0:20 Red Chapter 3
0:31 Red Chapter 3
0:42 Red (x2) Chapter 4
1:02 Purple Chapter 4
1:30 Purple Chapter 5
1:44 Red Chapter 5
2:14 Red Chapter 6
2:30 Red Chapter 6
3:10 Yellow Chapter 7