How do I feed a custom property from an object to a node of that object's material

Let’s say I have a sphere, let’s call it sphereA, with materialA assigned to it. MaterialA has an RGB node that I want to change using a custom properties stored in sphereA, using drivers. I have three custom properties in sphereA, one for each RGB color. The “single property” option doesn’t work for me, because I am using materialA with other objects too… so I’m not entirely sure this is even possible.

I tried self.parent[“colorR”], self.parent[“colorG”], self.parent[“colorB”] but that didn’t work. Any ideas?