Different Bees

In our outyard- 7 hives- we've noticed that 2 of the six hives have darker, almost black bees. Also, the darker bees seem more defensive. Is there really a difference?
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Different colors? Sure. Color doesn't necessarily dictate temperament though. At all. I've had lots of very dark NWCs for example that were exceptionally gentle, and also very light lemon bees that looked Italian to me that were nonetheless meaner'n snot. And the other way around too. Do you know what race your queens are (especially the hot ones)? You're in a potentially Africanized area, so if there's any chance those colonies swarmed or have otherwise incorporated local genes, I'd consider requeening ASAP with known stock from a reputable breeder.

In your area they could have some genetics from German black bees, which are a very defensive bee. Then again they could have genetics of Carniolan or Russian bees. All depends on which races of drones the queen mated with. Some hives will have a couple different colors of bees all the time, some will have a different color for a while depending on which sperm fertilizes the eggs. Don't worry about Africanized bees; if you ever get them you will know immediately by the overwhelming defensive behavior; then requeen.

Thanks for the advise. I've only purchased Italians, but have allowed them to make their own queens. I'm leaning toward the possibility of German genes. I know some bee keepers in our club have started keeping them and say they are very aggressive "mean." There aren't any close enough that I know of, but that doesn't mean there aren't some in the area that I don't know about. Some days I work the "hot" hives and they are fairly calm. Most days they are aggressive and fly at me after being in the hive for only a few minutes (lots of bees in the air and smoke does not seem to calm them much). One or two usually follow me a hundred feet or so from the hive then give up. I plan on re-queening from a separate yard (more than 2 miles away) after our honey flow. If that doesn't work, I'll consider purchasing new queens from one of our suppliers next spring. They are not much fun to work.