[MiNiGuY]

ok - ebrit, I looked into this yesterday for you.

Here2help is correct in that the discs are to do with the battery, but the colours have nothing to do with whether or not a car has a flat battery or not.
They are there to indicate whether a battery needs checking or not, although it's possible that people like here2help still use the disks but in a different way.

At the factory it works like this:-

There are six colours used at any one time in the disk system.

Currently they are Red,Blue,Green,Yellow,Brown and Grey.

White is currently not used.

For each full week of production, every car made is given one of these colours. For example, all MINI's built this week have had a Red disk placed onto the rear view mirror.
Next week it will be a different colour and so on for a six week cycle. Then it's back to red again.

Basically it is then possible at a glance to see which week in the last six any particular car was made. From this it is possible to quickly see if that car requires a battery check and subsequently a possible recharge.

That's basically it. Simple really.