Hi,
Can you create a new scene, and a new material. and work from this. It's likely that in your scene, you may have conflicting behaviors, all trying to set the color of the same material. Or you could be editing one instance of the prefab, instead of the prefab itself, which would result in trouble and inconsistency.
So, make sure that even the simpliest cube and material as a prefab behaves normally and we'll take it from there to solve this issue.
Bye,
Jean