Don't know how true this is but I read a number of years ago.....
A well-know tyre company (can't remember what one) made a series of tyres that had different coloured tread grooves in them, only available in the US or through import. Like where the groove for the wear limit is, except red, blue, yellow etc. Personally I quite liked them, very subtle but different and the colour remained through the life of the tyre.
Anyway.... apparently they were outlawed in the US because gangs where using the tyres as gang colours and committing felonies with them :S