My guess with the Aleppo--presumably Syrian-- za'atar would be that it is redder because of more sumac. I'm pretty sure it was in one of Claudia Roden's very through Middle Eastern cookbooks that I read that Syrian dishes, in particular, often have a sweet/sour interplay that isn't as common in other parts of the Middle East--which an extra jolt of sumac would certainly help to achieve. Syrian dishes use more pomegranate molasses (again, that sour note, but also sweet) than dishes from other Middle Eastern areas, too. (except for Persian cooking, which uses a lot of pomegranate molasses, too.)
Apparently za'atar is one of those spice mixes often made at home with lots of regional and often closely guarded familial variations. It seems to be mostly a Levantine condiment--probably because that's where the thyme and sumac grow wild, though I did used to get Za'atar spread flat breads at the Iraqi bakeries in Detroit when I lived there. The Za'atar that I buy is mixed at the market where I buy it. They are Palestinian and the za'atar is quite green and heavily herbal. There's sumac in it, but you can't really see it. It's there but it's more subtle than other za'atars that I've had in the past.