Whilst these reasons may well be valid, you have overlooked the obvious. As red has the longest wavelength of any of the visible colours (and you wouldn't want to pain it in an invisible colour now would you!!!) this effectively reduces the number of wave sines emmitted by the vehicle. Whilst the average landy has enough power to propel it up a hill, it is touch and go, and is only made possible by the tiny increase in propulsion provided by the millions of waves of light emanating from the vehicle. Whilst these individually provide very little difference, there are so many of them that it just tips the landy in favour of going up hills...except the red ones, which, as the wavelength is longer, emanate slightly fewer waves for your money, which is why they fail on hills.
Oh, and they're ****e too. Except Disco girl's, which was clearly fitted with a better engine than most.