GB One Penny Red - How To See The Difference?

Hi all.
Regarding red one penny perforated with the same watermark and perforation - how to tell the difference?
I refer to SG 29,30,32,33,38-42.
Thank you for reading.

29-33 are on blueté paper. This has a bluish tinge. The others are on an off-white paper lacking that tinge. An example is shown in this thread.

http://goscf.com/t/79466&whichpage=1#719629

There is a lot of discussion on whether the blueté paper actually existed. The Prussiate of potash in the red die reacts with liquids and can colour the paper blue. Sometimes, you can see this from the back when a kind of blue frame appears.

Between the groups, these are "shades," and this requires comparison / experience.

So basically the perforated one penny red versions are different in 3 ways: white and blueish paper, perf 14 and 16, and the rest is the color variation? I read in the forbidden site that in SG 42 "Letters engraved on plate instead of punched".

BTW, your link refers to unperforated stamps, if I read it correctly.

SG42 also has alphabet type IV, whereas the others have type III. All are die II, large crown watermark, perf. 14 (unless the SG numbers have changed, that I doubt happened in the last 50 years for these issues).

And yes, the link shows a penny red next to a penny black with the good difference in the colour of the paper.