Well, I managed to solve it like this:

    actor = (scene.Actor("hutRoofLP_19"))       
    mat = actor.Materials()[0]
    # find the material's shader tree and its root
    tree = mat.ShaderTree()
    root = tree.Node(0)
    diffCol = root.Input(18) # <<< this is the Alternate_Diffuse
    diffCol.SetColor(1,0,0)

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