I would not recommend using .vtk file format for medical images. The limitation that instantly disqualifies this file format from serious use for medical imaging that it cannot store image axis directions (it always assumes image IJK axes are the same as patient LPS axes). Also, the format can only store 4D images (so you, cannot store a 3D+t color volume) and there is no way to specify what the 4th dimension contains (color, displacement, etc.). Finally, we already have 3 other widely used file formats (nrrd, nifti, and mha), so you really don’t want to complicate things with bringing in yet another one. ParaView can read/write .mhd and read .nrrd, so if you use ParaView a lot then you may prefer to use .mhd file format.
You can of course convert any image formats to anything else using Slicer using the GUI (load the data set into Slicer then choose a different format when you save the data); or running this Python command:
but it makes your life simpler if you use the same file format across all software in your workflow.