I guess it has to do with the fact that everything is greyscale nowadays ... did you notice there are no more coloured cars, and the few that are there are all faded.
The cars are primarily an efficiency thing; it's more efficient to just make a few colours. The requirement for those colours to be boring _may_ be coming to an end, though; see the Renault 5. It only comes in five colours, but two of those are yellow and green. Anecdotally, pretty much every Renault 5 I see around is yellow or green, so apparently people like it.
I'm guessing limited car colors benefits the manufacturers more so than the customer. Less choice means less cost. Additionally, by avoiding 'polarizing' colors, they likely won't have issues with unsellable stock.