Tweak a single object

Drag'n drop from the Node List into the RenderGraph, add a new Surface2 shader, and change the Diffuse Color.

Select the object you want to tweak in the NodeList (right bottom tab) and drag and drop it in the RenderGraph View.

Create a new Surface node using the Node Picker (Ctrl+Space) and typing 'Surface'.

Connect the sphere to the new Surface node and drop the output of the surface node on the original Surface node output connection.

A new node is created. Set its mode to "override". You can now tweak the new surface node, it will affect only the object you chose.