Im fairly sure they had yellow

Their wookieepedia page has this picture as well as a detailed description of the answer your looking for. They have yellow plasma blades.

I don't really recall seeing the guards in Phantom Menace or Revenge of the Sith, unless I haven't been looking at the right sceneries? As far as I'm aware, my only exposure to the Jedi Temple Guards was during that Clone Wars arc where Ahsoka is taken to trial, and a group of them tried to stop Anakin and Bariss Offee from fighting, "Cease hostilities!" And of course there was the Grand Inquistor's backstory somewhat revealed in a Rebels S2 episode.

The temple guards had yellow lightsaber pikes. I think your cousin meant that Jedi guardians had green lightsabers which is correct.

Well I don’t remember seeing them in the movies, but in Clone Wars and Rebels they had yellow lightsabers