Chameleons book series

[Tiggles]

Does anybody know anything about this series of books?
DS has brought a couple home from school and really enjoys them. I've finally found them for sale on the net, (took ages as kept getting books about chameleons) but it isn't clear if there is an 'order' to them, in terms of are different colours of chameleon different levels of ability?
I don't want to buy any that would be too easy.
Does anyone know what their colour order is, or are all books about the same level?