The colour trials are popularly known as the rainbow trials. They start in March 1840, for selecting the colour of the stamp. The first trial plate had only three impressions.

Red		Blue

First trial plate of three stamps (DP19)

Image source British Postal Museum from the R M Phillips Collection. Vol VIII Pg 2.

A second trial starting in May 1840, when the penny black was already in circulation from 6th May. It was for experimenting with printing inks, cancellation ink and different papers. The stamps for these trials were printed in blocks of twelve, they had voided corner and no corner letters. The trials continued until Nov 1840. The work on the second series of trials resulted in the stamp colour changing from black to red in Feb 1841 and the cancellation from red to black as well as a new ink for the 1841 two pence stamp.

Second trial plate of twelve stamps (3x4) DP20

Image source British Postal Museum from the R M Phillips Collection. Vol VIII Pg 6

The first plate only had three impressions and was probably made in March 1840.
The second plate of twelve impressions was used from May to Nov 1840.
They were used for experimenting in printing ink, cancellation inks, and papers.

The colour of the first stamp issued was black

plate 1a	plate 1b	plate 2	plate 4

The second two penny stamp issued two days later was in Blue
and remained blue for 39 years.

plate 1	Die I, plate 1, Alph I	Die I, plate 2, Alph I

The work on the second series of trials resulted in the stamp colour changing from black to red. The Penny Black was changed to red in Feb 1841 and remained red for 39 years. The stamp colour was changed so that the black cancellation could be more visible and could not be washed off. The post office being afraid that people would try to reuse the stamps

Red Maltese Cross May 1840,		Black Maltese Cross Feb 1841 until 1844

The trials also helped establish the ink for for the new issue of the 2d in 1841

For the small trial twopence plate (DP43) go here

Colour trials were also carried out in 1876 for the 4d plate 15

High Value Colour Trials 1878

For even more Rainbow Trials see Vol VIII of the R M Phillips Collection

For more info see: Rainbow Trials by David Rowse. Over 150 colour photos, many from the Royal collection, much fascinating historic (letters, personalities) info on the year 1840. Vera Trinder Ltd