I name my betta's after the gems or colors that they match most closely. Diamond is my white halfmoon who has bits of yellow in his bottom fins and he started the trend because I could just not think of anything better. Which is how I came up with Amethyst, Sapphire (Illusion), Marble-Jade and Jasper (Steel) after.
Just makes me wish I could find a red I liked so I could have a Ruby or Garnet in there.
For the "other additions" to my tanks I went with names of characters from a game I've been involved with because I'm a dork like that.
Male Bettas: Jasper (PK), Gold Dust (PK) Iced Steel (EE HM)
Other: Faith, Frankie, Sonic, Ashley & Anna (Juli cory catfish), Mr. Bristles (albino bristlenose pleco), The Shrimples (dozen ghost shrimp)