I think those rules are equivalent to "when the crystal bag has exactly one crystal left, reset it to 1 crystal of each of the 5 colors".
So there's essentially a hidden 6th preview slot (the bag), with an undetermined yet not completely random color. As crystals are drawn, their color is crossed off. The number of possible colors that the next crystal can be always cycles from 5 to 2 and then resets.
This wouldn't be much fun to track by hand, but it might be nice to have a subtle little indicator somewhere - probably just one (lit or unlit) pixel of each color. Though I don't often look more than 3 moves ahead anyway...