Buses run on diesel. Trams will run on electricity.
Burning diesel in engines pumps particulates and other pollutants straight into city streets, causing severe pollution. Buses idling in traffic jams etc produces even more pollution.
Burning coal and oil for electricity does not produce city centre pollution. It still does pollute, but in a different way, but is a far more efficient way of converting energy than a diesel engine.
Electricity can also be generated renewably. The SNP is committed to boosting Scotland’s renewable energy output, and we have massive potential to do so (without resorting to windfarms in Perthshire).
And by the way, it doesn’t matter whether a bus runs on biodiesel, LPG or even olive oil – it still takes ages to get across town, gets stuck in traffic etc.
Painting them bright green makes no difference either.