Thank you for taking the time to reply and explain.
I also traced the earliest reference to the triune classification of prakriti to the Upanishads, basing myself on Radhakrishnan and some old papers by the early samkhya researchers like J.A.B. van Buitenen and others. I further consulted a relatively recent critical study of the development of Samkhya philosophy by Dr SGM Weerasinghe of Sri Lanka (which I found to be quite useful) and another older work on its origins by Sri Pulin Bihari Chakravarti., I went through a translation of the Yukti Dipika as well hoping to find something useful.
Further, I wrote to Dr Gerald James Larson, author of a study on classical samkhya and an editor to Vol 4 of the Encyclopedia of Indian Philosophies (Karl Potter ed) devoted to Samkhya, which I had studied earlier. He replied to me saying, and I partly quote, "The term "guna", of course, is dealt with in different ways by Samkhyayoga and Nyaya. Also relevant is the Buddhist distinct between "dharma" and "dharmin" which comes closer to Samkhya usage than to Nyaya usage. These are all huge issues and require careful discussion, since intellectual history in the early history of the subcontinent is lacking in adequate resources and the sources that are available are often unclear and lead to endless speculations. There are, unfortunately, no simple answers to these issues, and the only useful method is to read the ancient documents bit by bit as have folks like van Buitenen and Frauwallner. Good luck in your studies. If you find especially helpful new insights, do let me know."
Now, I did not find anything new because my analysis so far has only been tertiary (in that I just went thorough secondary analyses of primary sources).
Coming to the Upanishads, they are more in the nature of compendiums/records of theories rather than expository works. In my view, the red-black-white analogy is a bit like the old examples such as the billy goat-nanny goat analogy, two birds on a tree (one aloof and other eating fruits) analogy and the like. These analogies are useful, but I am not clear as to their origins.
I am not capable enough to sit in judgment about the Upanishads and inanely ask "where did they get this <specific theory> theory from". The only answer that suggests itself to me re this is "drishtam" (direct perception or vision), and that I think falls under the territory of Yoga (dhyana).
So I am stopping my inquiry here.
with kind regards
Harsha