The difference I could see after the fact was that while the leaves seemed to be packed into the stainless pot, the canning jars were much more firmly packed. Because of the way the rims of canning jars are, the leaves were held under the water more easily and there was definitely less surface area of the water exposed to oxygen. The stainless pot measured 8 litres but the canning jars were simply 3, 1 litre jars. While I don't know for certain, I think that the less air exposure was probably a big factor. As well, I wonder if it was easier to keep the temperature steady and high enough in the smaller canning jars than the larger stainless pot. I had a thermometer in the water vessel each time and with the canning jars, there was virtually no fiddling with the heat source temperatures compared to the stainless pot experiment.
Since the rest of my procedure was the exact same and the only differences were in which vessel I chose to heat the leaves in the first place, I can only think that it was the mitigating factor in the colour or lack thereof. Considering there were two vats in using each method and each time the 8 litre pot rendered less colour than the canning jars, I'm only going to use the canning jars from now on.
I redid the canning jar experiment the next day. I again got lots of colour and ended up letting the vat go because it was just too late in the evening for me to keep it up. I ran out of the white rovings I'd been using and put in some blue I'd dyed with indigo earlier this summer, thinking it might add a tad more colour to it. Instead it turned it greenish.. I totally wasn't expecting that!
There is much more Dyer's Knotweed to use. I've 1/2 lb of off white Bluefaced Leicester rovings that I can use and then I'm stuck with using white fleeces for the rest of this.