Steps to reproduce:

1. Open attached project "ProjectorMaterialBug.zip"
2. Open scene "scene" and notice that there are three projectors in the scene
3. ChangeMaterialProperties game object has Set Color script attached which uses Projector.material to edit material of one of the projectors
4. Run scene
5. Notice that all of the projectors start to emit green light
6. Exit play mode and notice that projectors stay green

Workaround: Create an instance of the original material and then apply changes to the instance rather than original material.