Well, you are relying on the memory of someone who was young and watched in horror as she put all manner of ill into the mixing bowl. It may very well have been beets. All I know is that the stamppot had a reddish tinge to it that left credence to the thought she was trying to feed me squished mice. I'm sure rhubarb would work.Awesome! I have been wondering about how to add some Turnips and Rhubarb to some of my meals. I bet some other root vegetable would work well in this as well.
Colcannon is very similar to the recipes I briefly looked at for Stamppot Boerenkool and mainly saw some different spices or flavorings added. Your grandmother's version reminds me of some of the East German recipes I remember from my days in that part of Germany.