The Doppler effect
does change the colour of a bus, but if it's going away from you it'll change to a darker red, not yellow. However, the difference in colour is so miniscule at everyday speeds, that it's essentially undetectable. It's about a millionth of a percent difference in light wavelength.

It however, buses were to travel at close to the speed of light, the red shift would be more significant, and you would see a colour change (for the very short period in which the bus was close enough to see!)

I'd also be more likely to use buses in that case, as I'd actually get to where I was going in a reasonable time.
( K2k6 has a proper job these days, Tue 29 Jul 2008, 15:13, Reply)