Here is the full list of all 18 Spiral Splatter achievements.
Congratulations, you've completed all levels
Congratulations, you've completed all levels with at least 1 star
Congratulations, you've completed all levels with at least 2 stars
Congratulations, you've completed all levels with 3 stars
Congratulations, you've unlocked stage 11
Congratulations, you've unlocked stage 10
You have splattered 500 times
Congratulations, you've unlocked stage 9
Congratulations, you've unlocked stage 8
Congratulations, you've unlocked stage 7
Congratulations, you've unlocked stage 6
You have splattered 250 times
Congratulations, you've unlocked stage 5
You have splattered 100 times
Congratulations, you've unlocked stage 4
You have splattered 50 times
Congratulations, you've unlocked stage 3
Congratulations, you've unlocked stage 2