I don't know how the Rwanda Rainbow got started but the seeds are a variety of colors and they are all solid colors. No patterns or markings. I would bet that all the plants in the mix are probably closely related as the colors in them all seem to have that same tone. Seed shape is all the same and sized the same. Every once in awhile there seems to be a variety that naturally produces several seed coat colors.
As I believe I mentioned earlier, one of the Rwandan Rainbow seed colors (a sort of unusually strong kidney bean maroon) seems to also be present in some small off type seeds that showed up in my packets of Speckled Grey from Richter's (which are Ugandan). Though clearly not identical (mine matured around June so it sounds like they are a different day length) they are likely related*.
Polychromatic beans seem sort of common in the Mid-African material. When I first got Fort Portal Violet, it was called Fort Portal mixed, and had shades ranging from maroon to browns tans and olives. The purple color (which in fact was only present on ONE bean in the initial growth) only showed up on all of them after they had grown (what happened to the other colors, or why they never seemed to come back, I have no idea).
Same thing with my Bantu beans, dozens of colors in, only purple out.
On the other hand, mixes seem to be common as well. Speckled Grey SEEMED to be a variable bean when I got with it's distribution of two main colors (solid black/purple and the Pebblestone like pattern) on secondary (black with white specks, which is sort of what you might expect a color reversal of the Pebblestone to look like) and a few outliers (which were probably other ones that had gotten in by mistake) But as each one seems to only throw back it's own color/pattern, I came to the conclusion that it is a mix with each separate**.
Even Fort Portal Violet is still a little of a mix since there is also Fort Portal Violet Extra which is slightly different (bigger plant, extra seed per pod (four as opposed to three***) and slightly bigger seed.
*Incidentally what color flowers did the Rawandan Rainbow had. One of the ways I figured out that the red ones were something different is that their flowers were white, as opposed to the purple of the others.
**Actually I knew that when they first sprouted. The black ones had strong cotyledon mottling (like Fort Portal Violet) while the speckled ones don't.
*** It's consistent so it's likely genetic.