“Obviously the Japanese eye is see the color differently than to foreigner. To you is is green, but to us it is called ‘blue’ or ‘aoi’. The blue means, ‘sorry, you will have to be out in the blue coldness for a while longer’ and the red color means, ‘welcome to the warm inside our taxi!’”
More seriously, I’d always assumed it was because red gets your attention, but it still seems silly to me. Alternatively, maybe it is red for warning, getting you ready for the shock of how much you will have to pay.