Here is the full list of all 15 Ink'n'Think achievements.
What is this button for?
Stain a button
Get the golden key
Get the lilac key
Get the red key
Complete all levels
Collect all the keys and find out how to use them
Complete 100% of the game
Paint most of the level green
Paint most of the level pink
Paint the same amount of the level with both characters
Score 100 points with any character
Score 200 points between the two characters
Switch between characters in solo mode
Play with the two female characters