Here is the full list of all 13 Pixel Art Coloring Book achievements.
Color 10 hard pictures.
Color 10 medium pictures.
Color 10 easy pictures.
Color 3 pictures without using boosters.
Color your first picture.
Use Tornado 1 time.
Use Bomb 5 times.
Use Magic wand 5 times.
Use Color splash 5 times.
Open the game
Color the entire Animals category.
Color the entire Architecture category.
Color the entire Food category.