Thanks again to those who replied. I think I have at least worked out the problem if not the solution - Poser does not regard folders as having individual settings, therefore only one folder icon is required. If you could change that colour, it would change all of them!

This would not be an issue if there was some standardised way of naming folders amongst the community that (for example) placed a single identifying letter at the front of the name to clearly identify the category to which it belonged. Yes, I realise that many vendors place MAT in the name but this still does not tell you whether it is clothing MATs or a V4 texture.

I will explore the idea of creating new folders within my existing runtimes, but that could mean almost rebuilding them from scratch so I'm loathe to do that if there's any other solution.