Jeff,
Yes, nymphs of specific insects do come in specific colors. These colors, or shades of colors, can also vary from stream to stream. Many pattern books will list specific ties which mimic specific nymphal stages of most mayflies.
You mentioned the Hares Ear, and Pheasant Tail. These are two pretty universal patterns. You are probably aware that they can be tied light, or dark in shade. While they are not 'imitative' in the sense of accurately portraying any specific nymph they catch a great many fish on a regular basis. They are suggestive of life and are therefore accepted by the fish as a potential food source.
The Orvis fly pattern book lists quite a few imitative nymphal patterns. If you 'Google' for example "e Dorothea nymph fly pattern" (sulpher) there are many articles about the fly and its stages as well as links to patterns which are pretty specific to that fly. Searching "Sulpher Nymph" may yield different results as E Dorothea, E Rotunda, and E Vitreous for example, while each being different, are many times lumped into the general "Sulpher" genre of fly.
Entomology is an interesting field in itself, but to the fly fisher who wants to fully understand his quarry and what it is likely to be feeding on it is essential.
In my fly boxes, to answer your immediate question, you will find some stream specific patterns as well as generic 'light and dark' nymphs as well as pupa. If I fish any stream on a regular basis, sampling what is there is a great way to learn what tweaks may be necessary to a particular pattern for the stream. If I am just here for the day, I will turn over rocks etc, and select a fly which most closely imitates what I find.
And don't forget the soft hackles, Partridge and Orange, Partridge and Yellow, Partridge and Green, etc. These have saved more than one outing. They are easy to tie, so you can get all you need quickly, and they catch fish like a magnet. They do not imitate anything specific, but they work almost anywhere you will fish.
Tight lines,
George