The Prussian Blue
According to Adrian Roose also at Stanley Gibbons, "On January 17th 1935 the final design was presented to The King for his final approval, with one question...what colour did he want the stamp to be? Given the choice of 'prussian blue' or 'blue' The King opted for the latter."
However, things did not go exactly as planned.
To learn how the 2½d Prussian blue (shown above) became one of Great Britain's rarest stamps, click here.