Why is it that when I playback XAVC-S 4K files in VLC media player the colors look clean and fine, whereas when I play the same files back in other programs like Sony Catalyst Browse they look muddy and dark?

Maybe VLC has some auto-proxy playback thingy that other programs don´t?

Of course none of the programs I have can play back the files fast enough to avoid massive stuttering but if I understand right that is because of my old faithful 2012 Macbook Pro´s lack of processing power not the programs.

What I would really like to know is if and when I do upgrade to a faster computer will the colors of the 4K files then look as good in the other programs as they do now in VLC - or is it just that VLC is somehow auto adjusting the files on the fly so they look better than they actually do?

It seems like you are viewing video without the broadcast level limiter on. This is what VLC is famous for. All other programs in your experience seem to be expanding 16-235 to 0-255. VLC always plays videos by default in Windows at 0-255, so footage shot with a 16-235 broadcast level limit will look washed out.