I just think it was long overdue to be seen on screen.
If it weren't for skyline issues in Jedi, a green one probably wouldn't exist...
Anyone who had a Luke action figure as a kid in 77-79 should know what I'm talking about.
It's kind of an homage to that.
Besides, Maul always had a red blade.
Vader/Anakin had a blue saber and lost it and then got a new red one.
Dooku already had a saber and would have no real reason to throw it out, especially for something that would scream his affiliation, if he was trying to operate in secret.