In testing the visualization with VTK, I made a simple example to visualize scalar data on a vtkUnstructuredGrid comprising of a single hexahedron cell. The code is below

import vtk

# Create the 8-node cube (hexahedron)
points = vtk.vtkPoints()
points.InsertNextPoint(0, 0, 0)  # Node 0
points.InsertNextPoint(1, 0, 0)  # Node 1
points.InsertNextPoint(1, 1, 0)  # Node 2
points.InsertNextPoint(0, 1, 0)  # Node 3
points.InsertNextPoint(0, 0, 10)  # Node 4
points.InsertNextPoint(1, 0, 10)  # Node 5
points.InsertNextPoint(1, 1, 10)  # Node 6
points.InsertNextPoint(0, 1, 10)  # Node 7

# Create the hexahedron (cube) with 8 nodes
# hexahedron = vtk.vtkHexahedron()
hexahedron = vtk.vtkLagrangeHexahedron()
hexahedron.GetPointIds().SetId(0, 0)  # Node 0
hexahedron.GetPointIds().SetId(1, 1)  # Node 1
hexahedron.GetPointIds().SetId(2, 2)  # Node 2
hexahedron.GetPointIds().SetId(3, 3)  # Node 3
hexahedron.GetPointIds().SetId(4, 4)  # Node 4
hexahedron.GetPointIds().SetId(5, 5)  # Node 5
hexahedron.GetPointIds().SetId(6, 6)  # Node 6
hexahedron.GetPointIds().SetId(7, 7)  # Node 7

# Create an unstructured grid
unstructured_grid = vtk.vtkUnstructuredGrid()
unstructured_grid.SetPoints(points)
unstructured_grid.InsertNextCell(hexahedron.GetCellType(), hexahedron.GetPointIds())

# Create scalar data
scalars = vtk.vtkFloatArray()
scalars.SetName("Node Scalars")

# Assign a scalar value to each node (for illustration, values 1 to 8)
vmin = -0.3
vmax = 0.0
scalars.InsertNextValue(vmin)  # Node 0
scalars.InsertNextValue(vmin)  # Node 1
scalars.InsertNextValue(vmin)  # Node 2
scalars.InsertNextValue(vmin)  # Node 3
scalars.InsertNextValue(vmax)  # Node 4
scalars.InsertNextValue(vmax)  # Node 5
scalars.InsertNextValue(vmax)  # Node 6
scalars.InsertNextValue(vmax)  # Node 7

# Add scalar data to the unstructured grid
unstructured_grid.GetPointData().SetScalars(scalars)

# Create a lookup table for the colors
lookup_table = vtk.vtkLookupTable()
lookup_table.SetTableRange(vmin, vmax)  # Set vmin and vmax to the scalar range
lookup_table.SetNumberOfColors(256)  # Set the number of colors in the table (256 is common)
# lookup_table.SetColorSpaceToDiverging()
lookup_table.Build()  # Build the lookup table

# Create a mapper
mapper = vtk.vtkDataSetMapper()
mapper.SetInputData(unstructured_grid)
mapper.SetScalarModeToUsePointData()
mapper.SetLookupTable(lookup_table)
mapper.SetScalarVisibility(1)
mapper.SetScalarRange(vmin, vmax)

# Create an actor
actor = vtk.vtkActor()
actor.SetMapper(mapper)

# Create a renderer, render window, and interactor
renderer = vtk.vtkRenderer()
renderWindow = vtk.vtkRenderWindow()
renderWindow.AddRenderer(renderer)

renderWindowInteractor = vtk.vtkRenderWindowInteractor()
renderWindowInteractor.SetRenderWindow(renderWindow)

# Add the actor to the renderer
renderer.AddActor(actor)

# Set background color to white
renderer.SetBackground(1.0, 1.0, 1.0)

# Use the scalar bar to display the color range
scalarBar = vtk.vtkScalarBarActor()
scalarBar.SetLookupTable(lookup_table)
scalarBar.SetTitle("Node Scalars")
scalarBar.SetNumberOfLabels(5)

# Add the scalar bar to the renderer
renderer.AddActor2D(scalarBar)

# Set the camera position
camera = renderer.GetActiveCamera()
camera.SetPosition(3, 3, 3)  # Position the camera at (3, 3, 3)
camera.SetFocalPoint(0.5, 0.5, 0.5)  # Set the focal point (where the camera looks)
camera.SetViewUp(0, 1, 0)  # Set the view-up direction

# Set the size of the render window
renderWindow.SetSize(600, 600)

# Start the rendering loop
renderWindow.Render()

# style = vtk.vtkInteractorStyleTrackballCamera()
# renderWindowInteractor.SetInteractorStyle(style)
renderWindowInteractor.Start()

I also created a scalar bar to visualize the color mapping of the look up table. Surprisingly, the color plot on the cell does not match with the scalar bar as the image below

I would expect that if VTK interpolate the color linearly using the shape functions of the cell, the color should match with the bar. Does anyone have any hint which could go wrong here?