For the nymphs, the earlier Invarias (there is no Rotunda anymore - they've been lumped in with the Invarias) have a darker nymph than the later Dorotheas. They're also bigger - sz 12-16, while the little sulphurs are usually sz 18-20. It's not unusual for the nymphs to be a size bigger than the adults as well.
For the adults, as someone mentioned, sz 12-16 for the earlier sulphurs, and sz 18-20 for the later little sulphurs is typical.
Coloration of the adults varies by stream. Some streams have pale yellow early sulphurs, some are more orange. The little sulphurs are often a lemon yellow color. To help offset this, I tie some of my larger adult patterns with beaver dubbing that has both yellow and orange in it (I cheat). ;-)
The good news is, you usually don't have to go nuts with different colors, since color is the least important of the primary triggers (size, shape, color, behavior). Occasionally it does make a difference, but a decent pattern of the right size and shape, combined with a drag free drift will usually produce. A rusty spinner in the right size works nicely for the spinners of all sizes. YMMV.