Great Britain 1840 1d "Rainbow" colour trial (State 2c), SGDP20Great Britain 1840 1d "Rainbow" Colour Trial (State 2c), SGDP20 A very fine and fresh unused four margin example with original gum printed in black on thin white wove paper. Wonderful colour and the most desirable of all the "Rainbows" and a rarity. These trials were produced to test various combinations of stamp and cancellation ink. It has long been considered that the voiding of the corners was achieved by filling the grooves of the plate with wax.