In the UK, official taxis run by the council tend to be black. However, Derby – a city in the UK – introduced a new regulation in 2001 that all the official taxis should be yellow. Presumably this was to make them more distinctive so that members of the public would be more certain that they were getting into an official taxi rather than an illegal one. One of the taxi drivers – John Kirkham – painted his car yellow to comply but was then shocked to have his application for the renewal of his licence refused because he had used the wrong shade of yellow!
The council backed down after adverse publicity. If only they had specified the colour tolerance as part of their specification they would have had a much stronger case.
Another reason for choosing yellow could be safety. Many school buses in USA are yellow – presumably for reasons of safety. A number of regions in the UK have recently been trialling the use of yellow buses. Getting more school children to travel to School by bus is seen as being good for the environment and would free up the congested roads. In the USA more than half of all chidren travel to School by yellow buses according to the BBC – http://news.bbc.co.uk/1/hi/uk_politics/7610933.stm
The use of yellow for school buses in the USA was introduced in 1939. The colour – National School Bus Chrome Yellow – was introduced for reasons of consistency and also for safety because of its high visibility. Cost was also a consideration since manufacturers charged extra for special colours. In addition the black lettering was easily visible by contrast on the yellow background (as you can see in the picture below).
The approximate colour can be obtained by #FFD800 or [255 216 0] in RGB values.