Paint is a magical weapon that works as Thinner's counterpart.
It is a substance that can be used with a paintbrush for creation. It can restore objects destroyed by Thinner. It can also be used to create and give life to Toons, hence why all Toons are made of paint. Paint comes in many colors.
The wizard Yen Sid created paint as a counterpart to Thinner.
During his time in Yen Sid's workshop, Mickey mixed together Paint and Thinner, accidentally creating the seemingly indestructible Shadow Blot in the process.