Sulfurs tend to nectar at flowers with their wings closed, so usually you only see the undersides of their wings, like the photos immediately below. Both of these butterflies are females; you can tell with the yellow form, on the left, because of the wing margin with yellow "bubbles" in the darker portions seen faintly through somewhat translucent wings. The dot pattern on the underside of the wings of the butterfly on the right identifies it as an orange sulfur; the white color tells you immediately, then, that it's a female.
Occasionally, you'll catch the butterflies slowly fanning their wings open and closed or you'll catch them as they are beginning to fly - then you can see what the upper side of their wings look like. The male sulfur, on the left (with the neatly defined dark areas along the wing margins), is probably an orange sulfur, although he doesn't have a great deal of orange in his wings. Without the orange, he could be a clouded sulfur, which looks very similar but has no orange wash to its wings at all. However, the yellow on his wings is very deep in tone and I saw no other obvious clouded sulfurs, so I expect he's actually a yellowish orange sulfur. The white butterfly on the right is the female white form.
Very occasionally you happen to catch a butterfly in mid-flight. Then you get to see some of the interesting maneuvers that their wings go through as they fly, maneuvers that occur much too fast for our eyes to note them without the magic of the camera pausing the movement. Sometimes the movements are graceful, as on the left, and sometimes they aren't!
One way or another, I'm sure that you've seen sulfurs before - they are so common that most of us just overlook them without paying much attention to them at all. There are 8 different species here in south-central Kansas, though, and that's not including the accidental adventurers, so it can be fun to learn to tell them apart.
As far as theses orange sulfurs go, many different legumes serve as their larval food plants, including alfalfa and clovers. The abundance of their larval food plants probably goes a long way in explaining their abundance overall.
Take a look next time you see a "yellow" butterfly and see if it's an orange sulfur or, perhaps, some other sulfur species. If you've read (and you look) carefully, you may even be able to tell if it's a male or female!