Steps:

• disk_out_ref.ex2, all variables, Apply
• Adaptive Resample to Image, Nnumber of image: 4, Apply -- notice: we're coloring by vtkBlockColors.
• Switch representation to Volume. You get errors

ERROR: In /home/utkarsh/Kitware/ParaView3/ParaView/VTK/Rendering/VolumeOpenGL2/vtkSmartVolumeMapper.cxx, line 271
vtkSmartVolumeMapper (0x558d7e7bccd0): Could not find the requested vtkDataArray! 5, 1, -1, vtkBlockColors

This is reasonable. vtkBlockColors indeed is missing for the volume representation. However, changing the coloring array to Temp, for example, doesn't change the error message. The only way around is to color the view, create a new one and color by Temp before changing the representation type to Volume