wikipedia says this In many metropolitan areas throughout the UK, taxis are licenced by the local authority,[18][19] and in many places are required to be painted a certain colour.[20][21] For instance, hackney carriages in Bristol are blue, yellow in Derby, silver in Portsmouth and on the Isle of Wight. These restrictions do not apply to licences granted previously, and many taxis remain unchanged.
but it's not exclusive, other licensed cabs are different colours. Brighton's are turquoise and white.
In London (and probably elsewhere now) many of the traditional hackney cabs are no longer black - lots I've seen are multicoloured and carry advertisements for their sponsors.



