because there are rules - trying to keep things a wee bit scientific here, folks!
1. You have to be travelling. On the bus, in a car, in a traffic jam. Watching out of the window doesn't count (Mum, I'm looking at you! The yellow car you can see out of the back window doesn't count!)You need to be on a journey.
2. Parked cars only count once. If you see a parked car - and you can count dealerships - you can count it, but if you pass the same car again on the way back, you can't count it.
3. You can't count cars/vans which are wearing yellow as livery. (This is where lorries are left out) If a van/car is yellow because (for example) it belongs to the Council, it doesn't count. Plain yellow vans with lights on the roof don't count. If it looks as if it is yellow as a matter of happenstance, it's OK.
Taxis which are yellow do count, as long as it isn't the entire fleet - look and ye shall learn!
4. Gold doesn't count - it's metallic beige. If you see a metallic yellow, you will know!