Here is the full list of all 20 Paint it Back achievements. It takes around 35-40 hours to unlock all of the achievements on Windows.
Repaint all of The Archives
Reopen the whole gallery
Win 15 timer ribbons
Win 15 paint ribbons
Win 25 checkmark ribbons
Win 10 no-X's ribbons
Win 3 timer ribbons
Win 3 paint ribbons
Win 5 checkmark ribbons
Paint 25 Master pictures
Paint 20 Pro pictures
Paint 50 Normal pictures
Paint 20 Quick Fix pics using no X's
Paint 5 Quick Fix pics without lifting your brush
Paint 5 Quick Fix pics using no X's
Paint 1 Quick Fix pic without lifting your brush
Win 100 ribbons
Paint 5 Pro pictures
Demonstrate some artistic ability
Paint 20 Normal pictures