I base mine on personality, looks, or temperament. Hera because she's top fish in the sorority. Baby because she is small and was a Petco baby betta. Lacey is a black lace, dark bodied, transparent fins with dark edging. Opal and Aurora are sisters, marble crowntails and rather dainty. Ysbryd is my new crowntail boy, he was "white as a sheet" so to speak, very pale and colorless, but doing MUCH better. Shad (in my avatar) just because I've always loved the name.
My passed males (poisoned because of plastic tubs), Duckie had yellow lips, Amber had amber eyes, Poseidon was a beautiful mustard gas and very regal, Ares was a big blue/red dragon, and Zero was a frosty red dragon.
Three females still don't have names yet because I don't have anything that suits them.