I think you're seeing the males and females at different times..the males do look much more streamlined, but even with that, if you get a pair of binoculars and really watch them, you can often tell the difference between birds based in length of beak, curved or straight, color variations, etc..

Thanks for the insight, Donna. Some of the dusk visitors were males, all right (flash of red gorget).

I've noticed this too, and read into it a bit. It seems the colors and hues we see from hummers' bodies are more a product of an incandescent effect of the light hitting their feathers than the actual pigments. In other words, as the sunlight changes and hits them in different ways, they look different to us. Less light generally means less color to our eyes.