Hi everyone,  quick piece. Thanks for answers to Penny Black question. Here is an answer I received... Hope you enjoyed the piece I wrote...

Rowland Hill did the Rainbow and Obliterating trials to remedy some defects in the penny black.
Black ink on a black stamp doesn't show up so they needed a better colour for both the stamp and the cancel. There were people who were cleaning the stamps and reusing them. The trials tested different inks and colours to find the best combination of stamp colour and cancel. Hill's test found that a black cancel on a red stamp worked best. Introduced the Penny Red in Feb. 1841. Also the Penny Blue.

Keep reading if you haven't already read recent pieces..      

Enjoy your stamps..   Best wishes...Michael