I figured it out!
Choose your primitive containing the node
var primitive = viewer.scene.primitives._primitives[0];
View node names like this
Object.keys(primitive._nodesByName);
Choose the node to be colored (1st one in this example)
var nodeName = Object.keys(primitive._nodesByName)[0];
Change the node’s color like this!
primitive.getNode(nodeName)._runtimeNode.node.primitives[0].material.metallicRoughness.baseColorFactor=Cesium.Cartesian4.fromColor(Cesium.Color.RED);
I got inspired by these posts :
Changing material of single mesh in model
How to color specific part of Model