Want to access ScriptableObject data from an editor code, namely a custom Node Graph Editor code:
Create a ScriptableObject containing Color field.
Create an instance of that SO and set the Color in the inspector.
Acquire that SO instance from an editor script.(if this was a Monobehavior we could drag drop into a SerializeField, how to achieve this from an editor script?)
Use the color from SO instance.
You can use :
SO SOInstance = Resources.Load("PATH_OF_YOUR_SO");
https://docs.unity3d.com/ScriptReference/Resources.Load.html