There are a wide variety of theories as to why red was chosen. There is also a factual explanation.
At very start of the 20th Century, when the majority of vehicles were still drawn by horses, buses came in a range of colours. Red only rose to popularity when, in 1907, the London General Omnibus Company (LGOC) rouged-up its fleet to distinguish itself from the competition.
LGOC soon came to dominate the early PCV market; when London Transport was formed in 1933 it therefore chose to extend the colour to most of the capital’s buses. (The shade chosen, as the Londonist has pointed out, is the same used by McDonalds, the Royal Mail, Kit Kat and the Russian flag).
However, it is a myth that all London buses are red. Or rather, it is a myth that London buses are all red.
Indeed, when one breaks down the space allocated to the colour on buses, a different picture emerges. For example, the roofs of the vehicles are predominantly white – this reflects sunlight, and reduces heat in summer. It is also reasonable to assume that the undersides are not red, as that would be a fairly pointless paint job. And let’s not forget the area used by windows and adverts – the latter of which can take over vast swathes of a bus’ body.
Taking all the above into consideration, the Londonist calculated that a typical vehicle is only 30-40% red. If so, we can only agree with them that a famous British symbol is in danger of becoming a redder herring than a bus. Time, perhaps, for change.org. I feel a petition coming on.