I'd like to create a model that uses scalars for multi-color appearance. But the model appears all gray (no color). However, anything that causes the DisplayNode to update after the script, makes the colors appear. See below the shortest code I could write to reproduce the bug. Just paste it in the python interactor.
It's funny when I manually repeat the last line after running the whole script, the colors appear. So it looks like Slicer needs a bit of time before the displayNode.Modified(). (But a time.sleep() didn't help :-) )

import slicer, vtk

Create polydata with scalars.

s1 = vtk.vtkCylinderSource()
s1.SetHeight( 50 )
s1.SetRadius( 10 )
s1.Update()
p1 = s1.GetOutput()
a1 = vtk.vtkDoubleArray()
a1.SetNumberOfComponents( 1 )
a1.SetNumberOfTuples( p1.GetNumberOfPoints() )
a1.SetName( "MyColors" )
for i in range( p1.GetNumberOfPoints() ):
a1.SetValue( i, 60 )

p1.GetPointData().AddArray( a1 )

Create polydata with different scalars.

s2 = vtk.vtkCylinderSource()
s2.SetHeight( 20 )
s2.SetRadius( 20 )
s2.Update()
p2 = s2.GetOutput()
a2 = vtk.vtkDoubleArray()
a2.SetNumberOfComponents( 1 )
a2.SetNumberOfTuples( p2.GetNumberOfPoints() )
a2.SetName( "MyColors" )
for i in range( p2.GetNumberOfPoints() ):
a2.SetValue( i, 80 )

p2.GetPointData().AddArray( a2 )

Append two polydata into one.

append = vtk.vtkAppendPolyData()
append.AddInput( p1 )
append.AddInput( p2 )

Create a model node and add it to the scene.

scene = slicer.mrmlScene
modelNode = slicer.vtkMRMLModelNode()
modelNode.SetScene( scene )
modelNode.SetName( "MyModel" )
modelNode.SetAndObservePolyData( append.GetOutput() )
displayNode = slicer.vtkMRMLModelDisplayNode()
displayNode.SetScene( scene )
displayNode.SetName( "MyModelDisplay" )
scene.AddNode( displayNode )
modelNode.SetAndObserveDisplayNodeID( displayNode.GetID() )
displayNode.SetPolyData( modelNode.GetPolyData() )
displayNode.SetScalarVisibility( 1 )
displayNode.SetAmbient( 0.2 )
displayNode.SetActiveScalarName( "MyColors" )
displayNode.SetAndObserveColorNodeID( "vtkMRMLColorTableNodeRainbow" )
scene.AddNode( modelNode )

displayNode.Modified()