"The Rainbow Trials" refers to a series of historic stamp colour experiments conducted in Britain in 1840, not a specific set of stamps issued for public use. These trials were instrumental in the transition from the Penny Black to the Penny Red stamp.

Purpose of the Trials
The "Rainbow Trials" were initiated by Rowland Hill and other General Post Office officials to address a significant security flaw in the world's first adhesive stamp, the Penny Black.
The problem: The red cancellation mark on the black stamp was found to be easily removable using chemical agents, allowing the stamps to be reused fraudulently.
The solution: Experiments were conducted to find a combination of stamp and cancellation ink that would be more secure. Different colours and various "fugitive" inks (inks that would run or react with chemicals) were tested.
The outcome: The trials determined that printing the stamp in a colour that was difficult to clean, such as red, and using a black cancellation mark would make any attempt at reuse clearly visible. This led to the introduction of the Penny Red stamp in February 1841.

The States of Impressions of the Second Trial Plate
State 1: These are the initial proofs, appearing in black, orange-red, reddish-brown, and deep blue. They feature voided corner squares.
State 2: Almost identical to State 1, but with a specific, small mark added to the bottom right corner square of the second stamp in the third row. These are sometimes found on watermarked laid paper.
State 3: Distinguished by the rougher, more pronounced edges of the voided corners. In this state, the impressions were re-entered, and the corners were voided more carefully, resulting in smaller and more uniform, yet often "thinner" looking, corner squares.

This plate was used to test numerous inks (including prussiate of potash) and papers to find a "fugitive" ink that would destroy the stamp if the cancellation was removed.