From the Wikipedia page on Taxicabs of the United Kingdom [1] The Taxi and Private Hire office is the body responsible for licensing taxicabs within Greater London. Taxi and Private Hire is part of Transport for London and is responsible for licensing the familiar London taxicab or "black cab" and also licenses private hire or minicab services. "Black cabs" were traditionally coloured black, but this is not a requirement and cabs are painted in other colours, sometimes bearing advertising; however they are traditionally called black cabs to distinguish them from minicabs.