The etymology of "people of colour": an appeal

By my bedside at the moment is “Asians in Britain: 400 years of History” by Rozina Visram. It contains a reference to a line item in the accounts of the British Government’s Committee for the Relief of the Black Poor dated 1786, referring to “relief to the ‘Blacks and People of Colour'”(p. 21). This is the oldest reference to the term ‘people of colour’ that I’ve yet seen. Any suggestions in tracking down the etymology gratefully received…