William Hart had a predilection for intense colours, an attribute that would have pleased Gould. Though he commenced medical training, Hart was forced to abandon his studies due to lack of finance. He then turned to making his living as a colourer.
Commencing work for Gould in the summer of 1851, Hart made the patterns (master illustrations to be copied by the colouring workshop) for the humming birds and coloured the metallic parts of the plates. He also worked on The Birds of Great Britain with H C Richter.
By 1870 Hart was Gould's chief artist and lithographer, and frequently coloured the finished plates. During this period he worked on The Birds of New Guinea and lithographed 141 of Gould's sketches.
Following Gould's death in 1881, Hart was employed by Dr R Bowdler-Sharpe to complete Gould's work on The Birds of New Guinea. Hart's best work was on the Birds of Paradise in The Birds of New Guinea - their bright colours suiting his tendency to overcolour.