Following trials in 1840, a decision was taken by the Post Office to change the colour of the 1d stamp (and the colour of the ink used to cancel them) from black (cancelled with red ink) to red (cancelled with black ink).
This was enacted in February 1841 and the official date for the changeover was 10 February 1841. The design of the stamps remained exactly the same, and indeed the first 1d red stamps to be printed utilised some of the printing plates that had previously been used to print 1d black stamps.
In addition to those stamps printed in red from the “black” plates, an additional 166 printing plates (numbered 12 to 177) were used to print the imperforate 1d red stamps from 1841 until their eventual replacement with perforated stamps in 1853.
Although superseded by later issues, these stamps remained valid for postage until all stamps bearing the image of Queen Victoria were finally de-monetized in 1915.