Your questions provoke a "maybe yes, maybe no" answer.
Color and size for a species falls within a very general range. Color varies within that range from stream to stream, from male to female, and from dun to spinner. Size varies from stream to stream, from male to female, and (with extended hatches) from early in the hatching season to later in the hatching season.
Hatch timing for the same species in different streams or stream sections depends on seasonal temperature variations and the temperature regime of the stream. Streams with more stable temperatures (like limestone spring creeks) usually have more consistent and reliable hatch timing. Freestone streams are subject to broader fluctuations, both from season to season and from lower mainstem reaches into the headwaters. In either case, long-term temperature trends will affect hatch timing a little to a lot. (See the earlier discussions in the threads about "degree-days" and hatch timing for more.)
I know that isn't very helpful, but here are some specifics about the "sulphur" hatch(es) that might help:
Ephemerella invaria (includes the former E. rotunda)--This is the most tolerant, adaptable, and widespread of the "sulphurs." It is the main player in most of our region's streams. The larger female duns range from pale to bright yellow, often with secondary shades of olive or orange. Wings of duns range from very pale (almost white) to light bluish gray or medium milky gray. Males are smaller, darker, and have large orangish eyes. Male dun body colors range from tannish yellow to light or orangish brown. During the early part of the hatching season, females are typically in the #12-14 range and males in the #14-16 range. On streams with strong extended hatches, size diminishes to #16-18 for females and #18-20 for males. The hatch typically begins somewhere before or after mid-May, but that can vary widely from north to south, stream to stream, and season to season. On streams with heavy populations, the hatch can last well into June or even beyond.
Ephemerella dorothea dorothea--In much Eastern fly fishing literature, this species has been credited for the smaller, later "sulphur" hatch that overlaps or follows the invaria (rotunda) activity. Its habitat requirements seem to be narrower than invaria. Because of this, some of the activity attributed to this species may actually be extended invaria hatching. Female duns are typically bright to orangish yellow, sometimes with olive tones. Wings are very light to dirty gray, often with a hint of yellow along the leading edge. Male duns range from dark cream to tannish yellow with large red or reddish orange eyes. These little sulphurs seem to be more concentrated in slower water (pools) during the hatch. Hatching often begins in late May to early June, but varies with region, stream, and weather.
Ephemerella septentrionalis--This "sulphur," although fairly widespread, seems to have the narrowest habitat requirements of the three, and is probably the least likely to be encountered. It is fairly large, and because its hatching period overlaps invaria, it is easily mistaken for that species from a distance. However, the long spindly legs of both the nymph and adult easily distinguish it from invaria.
Other "sulphurlike" hatches:
Epeorus vitreus (sometimes misspelled as "vitrea")--This relative of the quill Gordon (Epeorus pleuralis) looks somewhat like a large sulphur from a distance, but is easily distinguished from them by the two tails (as opposed to three in the species above). The male duns are dull yellow with an olive or greenish cast. Dun wings are nearly white to very pale gray. The larger female duns (as large or slightly larger than invaria females) usually have bright pinkish orange or reddish abdomens reflecting the bright egg masses underneath. Because the hatching period overlaps the sulphurs, these "pink ladies" may share the water with them on suitable streams. Trout sometimes key into the bright vitreus females. Whenever I hear about hatches of "peach" or "coral" duns, I wonder if vitreus may be involved. Size is usually about #12-14, but later-hatching vitreus (into September on some streams) are #14-16. They emerge underwater, making wet-fly tactics important.
Other heptageniids--Some large to medium (#10-14) whitish or yellowish "Cahills" (mostly Maccaffertium species) may emerge with the sulphurs, but are easily distinguished by two tails and marked wings (usually dark hashmarks along the leading edge and middle of the wings). Additionally, some small, yellowish, "minor" heptageniids (mostly Leucrocuta species) might also be seen during the "sulphur" hatches. These are about the size of the dorothea or late invaria (#16-18). They have two tails, hashmarks on the wings, and often have brown markings on the back of their yellowish abdomens. The most significant of these, Leucrocuta hebe, usually follows the smaller sulphur hatches from sometime in late June, often extending into the fall.
I hope that helps, Casey.