I'm still using a black hemp back to store my core mid collection, and it can hold 6 or 7 colors if memory serves. So I use the 4 "classic" colors in the game itself, black and white mids to mark the koans and let people guess, and whatever color I have left as the guessing stone rewards. Means I don't have to carry around any stones and there is a nice symmetry that the pyramids are used for everything.