The thing with paraduns is how many variations will you likely need? It is far too easy to fill up your fly boxes with a myriad of different colours and sizes. Simplifying things to a basic level we can group olive duns into three sizes;
Large, (16) to imitate the large dark olive; round my neck of the woods anyway.
Medium, (18) will cover medium olive, large spurwings/pale watery, iron blue and blue winged olive.
Small (20) for small dark olive, small spurwings/pale watery.
With sizes sorted out what colours should we be looking at? Perhaps we should really consider them shades as they will all be some sort olive.
Dark for large and small dark olives.
Medium for the medium olive and BWO.
Light for the spurwings/pale watery.
The iron blue requires a colour of its own due to its very specific dark colouration.
So now we have decided what colours we need, we can now combine them with the relevant sizes and decide which variations of paradun we need to tie. I have whittled down my requirements to a large and small dark paradun, and both medium and light paraduns in medium and small sizes. I haven't come across the iron blue dun, but there is no reason they don't exist in the waters I fish, and for that reason it wouldn't hurt to have a paradun to cover that eventuality.
Other considerations when tying your selection of paraduns are wing and hackle colour. Again to keep with the simple theme you need only medium dun poly yarn and medium and light dun hackles; my own are medium grey dun and light brown dun. The iron blue again needs its own materials but a dark dun hackle and wing will suffice.
I know this extremely simplified, nevertheless I feel this would cover most
situations without leaving the angler at a disadvantage.