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.