counting cars

Trying to keep my attention on the road. Same journey every day - twenty miles there, twenty miles back. One day I realised I had no idea where I was - had I gone past Kimberley yet? Was I somewhere between Barford and Norwich?
I had no idea, so must have been driving on autopilot.
Must learn to pay attention.
Have hit upon trick of counting cars. Not all of them, but a different colour every day. The easy ones first: blue (75), black (54), silver (63) but hit an immediate problem - when does blue become purple? When is silver gold or white? In fact, is it a colour at all, since I counted metalic blue as blue, so should silver merely be counted as metalic white or grey?
It was really hard, but did keep my attention on the road. It also raised another question: why are most cars blue, black or silver i.e. the colours of uniforms? There were hardly any pink or yellow ones.
Answers on a postcard, please, but do you mind if I read it later, when I'm not driving?
Watch that lampost!