Another possibility is that she’s XY, but the Y never activated, so she developed female but with a single “faulty” X chromosome.
I don’t remember my biology classes well enough to say, but wouldn’t that also mean that potentially neither of her parents were colorblind, since the Y would’ve come from her father while the faulty X would’ve come from her mother? And, if she were XY in this scenario, wouldn’t that mean that she’d pass that trait along to her kids as well?
The original comic was rather popular at the time, and as a result, it became an early meme before mass-scale meme culture had really taken off besides doge memes and “I can haz cheeseburger.” So it quickly entered the cultural zeitgeist of the early internet because the kinds of people into memes and gamer culture at the time would’ve been about the size of the terminally online crowd today.