From forum thread : https://forum.unity.com/threads/how-to-change-decal-color-at-runtime-hdrp.635023/

The Decal Projector is not instantiating its own material when calling 
{noformat}
DecalProjector.material{noformat}
The behaviour should be similar to the Renderer apis:
{code:java}
Renderer.material
Renderer.sharedMaterial{code}
 

Where .sharedMaterial returns the original material, and .material will create an instance of the material if needed and return it.

This makes the code easier to write for users, and also avoid to manipulate the referenced material asset by error.