Can anyone explain ORT's (random?) level/colour pairings?

[Hooliesmoolies]

They probably aren't random, but does anyone know why they have stage 7 and stage 8 turquoise books, for example? I should just add these aren't old copies, but new ones.

Is there a difference between a stage 7 and stage 8 turquoise book?

When we had some of the older ORT non-fiction books (old copies, rather than old editions I think) that were band 6 (say), and turquoise or purple, I had the impression that they seemed rather harder than the orange band 6 books, but I don't really know if they were in terms (based on what the children were learning).

No real need to know, except curiosity and a vain desire to actually understand how it all works .

when the books were written they used their own levels/stages. Then the book bands were introduced and books which had been grouped into stages were re leveled and some are a lot easier than others and some harder. There is a list available through the ORT website somewhere with all the bandings on it and some are 2 or 3 levels out.

So no no difference, it is the book band colour that is relevant not the ORT stage.