The 'penny black' was the world's first official adhesive postage stamp. It was only issued for just over a year (1840-1841) when this black stamp was replaced by the penny red. Why was the colour change found necessary?

If I'm not mistaken, the Penny black had a problem because the cancellation was in black ink; therefore they printed it in red, so the cancellation mark could be more cleary seen.

Almost: the cancellation mark was originally in red, not black, but proved difficult ro read on a black stamp and was easily removed, too. So the colour of the stamp was changed to red and the cancellation ink made an indelible black.[Wikipedia: "penny black" ]

Boy's Own Paper for March 8th 1879 [ see, I keep all my old magazines ]suggests, in Answers to Correspondents, that the black stamp was replaced because it was easy to copy 'by photographic means'. Sounds a bit fanciful, does that, but who knows? Perhaps the official version was a cover-up