mmmm and that guys not been active for a while now
I did a search on youtube
https://www.youtube.com/results?search_query=how+to+disassemble+an+ultra+sabers+guardiantheres a load of review videos, sometimes in those people take bits off so you might be able to figure it out from those
unless someone with more knowledge about this saber than me answers the only thing I can suggest is if you carefully attempt to figure it out, I'd start with removing the pommel, emmiter and covertec wheel (if you have one). if the guardian has a shroud cover (which I believe it does) that should get it loose, but sometimes you have to remove the button to get these off
if its only the black part you want to paint, try using masking tape to cover the parts of the saber you don't want to change