Hi,
you need to change the material resource itself then.
use the "set material color" action. set the gameObject to none ( select the variable option, but don't select any, it shosws then 'none')
drag the material from the project browser ( NOT the scene itself), and then you will change the material itself and any object using ti will update.
If you have trouble setting it up, let me know, I'll do a quick working sample, but you should be fine.
bye,
Jean