We don’t have any geometry filter inside vtk.js and right now, you can only render a vtkImageData with VolumeRendering and vtkPolydata with GeometryRendering.
To allow what you need, you need a filter that convert your vtkImageData into a vtkPolydata by only extracting the skin of your dataset so you can render it as geometry with the coloring happening on the cells.
Such filter can easily be written for image data though. For other types of mesh it will be way trickier.