I used to know how to enter the html for that in guildnames box when I played test shards, then as far as I know the devs nerfed it a couple years ago.
Might wait and see if their guild name loses its color after server up, seems like some of the items we used the htmls on did.
We did neon colored guild names, books, house signs.
Seems on production servers every server up the guildname neon colors got wiped and you had to cough up the 25k to redo your guildname and reenter the html code to reset the color. That was expensive!
There are at least 2 old houses on Pac with the neon html colored house signs.
So my guess it's just an old char reactivated and server down will take care of his neon html guildname for him.
If it's a player you see everyday that means someone's figured out how to do the neon htmls again.
Possibly the last patch the Devs unwittingly removed or changed the part of the code the old devs put in place to block us making neon html named stuff?
Time to go do some testing....