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
Paint 25 Master 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 20 Pro pictures
Paint 5 Pro pictures
Paint 20 Normal pictures
Demonstrate some artistic ability
Paint 50 Normal pictures
Win 3 timer ribbons
Win 15 timer ribbons
Win 10 no-X's ribbons
Win 3 paint ribbons
Win 15 paint ribbons
Win 5 checkmark ribbons
Win 25 checkmark ribbons
Reopen the whole gallery