Of course one can't discount the wonders found on the Internet so I also looked up dyeing with prickly pear fruit there and the first hit was an incredible scientific paper "Eco-friendly and protective natural dye from red prickly pear (Opuntia Lasiacantha Pfeiffer) plant" from the Journal of Saudi Chemical Society.
Though waaaay over my head in detail, I was able to understand the summary of the results just fine. They tested for different mordants, ph, temperature, salinity, several kinds of color-fastness, time in dye bath, pre and post mordanting. And they didn't say, "Well, it looks a little darker if you do this." No, they used a Perkin-Elmer Lambda 3B UV/Vis Spectrophotometer and the Kubelka-Munk equation to measure color strength. I love these guys.
Tomorrow I venture back into the yard and pick the remaining ripe fruit to take back to Polson with me on the plane. I have just enough time to do a fermented batch before we head back to Austin. Note to self: Wave the fruit over a gas flame to burn off all the micro spines before packing.