I would guess there could be a number of reasosns firstly the edge of a tyre gets pretty dirty so they would look a bit second hand pretty quick. You mention team colors etc so I assume you mean racing as well, what happens in racing is that dunlop or who ever bring a truck load of tyres and the teams go to them and get what they want if they were in team colours then they would need loads and loads of the same tyre just to meet each teams requirements. Also they only last a race (if not half a race) and therefore the expense of producing coloured tyres would be a wast, OK with the exception of a crash a piece of body work with a sponsor logo would be expected to last all season.