I know the fuel gauge isn't very accurate but...

There's no such thing as a green Prius... have you seen pictures of the Tideland Pearl on the web, or seen the car in real life? The actual color is a medium-dark gray which can occasionally, in sunlight, at a certain angle, take on a subtly green sheen.

Have you seen 'Mystic Emerald' Nissan Altimas? It's a beautiful green/gray color; it's too bad Toyota doesn't use such a color in the Prius.

EDIT: Here's a picture gallery of someone's Mystic Emerald car. The green is subtle but not imaginary as it is with Tideland Pearl, and it actually looks even more green in real life.