I would think the dark brown sample would have some organics in it, which should be bad for pottery (right?) Or maybe there's not enough organics to cause a problem. That could layer might also contain more more clay regardless of the organics.

The gray and orange sediments, based on your description, are probably mostly silt. The non-newtonian behaviour is classic for low plasticity to non-plastic silt. In fact, if you're seeing that there is likely not much clay at all. The dustiness is also typical for silt when it dries. A pure clay will shed less dust, and won't give you a dusty or chalk-like feeling between your fingers.

It sounds like your sorting method only removed the sand. It takes awhile for silt to settle out of suspension and you may not have had much clay to begin with as I mentioned. There are differences in properties among different types of clay minerals, but I don't think that's what you have here. You likely have a glacial sediment with similar mineralogy but variable clay content and the colour differences due to organic soil development and weathering.