Watercolor does not expire, but it can lose quality overtime, especially if stored poorly. Overtime, the pigment in the watercolor paint will separate from its binding agent to the point where it can't be rehydrated again. Watercolor in a palette can last longer if properly taken care of.

How can you tell if oil paint is bad?

Rancid- or Sour-Smelling Paint

After the lid is opened, some paint might have a sharp smell: rancid, foul, or sour. Other paint might smell like mold or mildew. If the smelly paint is applied, the smell may lessen but not disappear. Bacterial growth is the driver behind this smell.

How do you know if tempera paint is bad?

If stored too long tempera paint can get a rotten smell. To combat this problem, be wary of adding too much water to paints upon storage. Also, don't order in bulk if you don't need too! Bottles (especially pump bottles) of tempera that sit around for years will have a rotten smell.