I agree that it's being misused in the sense that the original mustard gas bettas have dark (almost a black) smudges around the edges of their fins.
HOWEVER, it wouldn't be any different than someone trying to trademark any potentially random trait that would show up. That's like when Paris Hilton tried to trademark the phrase "that's hot". It's too random.
Without that smudge, I would call it a bi-color. But really by the end of the day, mustard gas is only a bi-colored butterfly. And it ISN'T trademarked, so being rude isn't going to make it any less legally moot than it already is, despite spending 15 years on it. That's the chance you take when you breed livestock, especially when you sell or give away without a contract.