I have no idea from where you have sourced your "freesurfercolorlut.txt` file, but it is clearly in a different format to that provided with FreeSurfer:
It seems to me that somebody has converted the FreeSurfer lookup table into a different column formatting, and changed the colours from 0-255 to 0.0-1.0 scaling. MRtrix3 doesn’t support this format because it’s not a format I’ve ever seen before (see “list” of supported formats here).
While it would be quite easy to modify the MRtrix3 code in order to support this file, it would still be worthwhile tracing the source of this file to know where it has come from, and the justification for formatting in this particular way (perhaps for compatibility with some other software?). Even if nothing else, knowing the basis of the format would give me an appropriate name for such within the MRtrix3 source code