The first consideration in selecting which nymph pattern to use should be the bottom composition.
With silty muddy bottoms or during high water periods after rain...Black rules! Steelhead guys who often face stained water love purple/blue uv highlights. Here in New England, a black hare's ear is a go to April fly.
With sandy bottoms you should consider tan hare's ear nymphs to be your first choice. Especially if your hatches consist of pale colored mayflies like cahills, sulfurs or yellow drakes.
Weedy bottoms calls for olive nymphes! I dye gray squirrel fur with yellow rite dye and a shot of vinegar to produce this pattern.
It's wise to have size 10, 12, 14, and 16's in the color the fits your situation. The bigger flies carry more weight which will get deep faster. Thank you for the view and please stay tuned. I plan to try to make sense out of the different european techinques, where and how they may apply here. Take care and let's go fishing!