If you have a full-color image then you can show a direct volume rendering of it by turning off “independent component” option in the volume renderer (it makes image components interpreted as RGBA). Type this in the Python console after enabling volume rendering:
getNode('VolumeProperty').GetVolumeProperty().SetIndependentComponents(0)
Note that you need an RGBA image (not just RGB). The fourth (alpha) component controls opacity. If you only have an RGB image then you can create the “alpha” volume using “Vector to scalar volume” module, then append this alpha volume to the original RGB volume by copy-pasting this to the Python console:
colorVolume = getNode('MyColorVolume') # RGB vector volume
alphaVolume = getNode('MyAlphaVolume') # scalar volume created from RGB volume using Vector to scalar volume module
append=vtk.vtkImageAppendComponents()
append.AddInputConnection(colorVolume.GetImageDataConnection())
append.AddInputConnection(alphaVolume.GetImageDataConnection())
append.Update()
colorVolume.SetAndObserveImageData(append.GetOutput())
For segmentation (if you want to create a surface mesh, 3D-printable model, etc), use the scalar volume created by “Vector to scalar volume” module.