In the bowl of a mixer, combine all the dough ingredients and knead until smooth and elastic.
The dough should form a thin and translucent membrane when stretched, like a "windowpane". (Kneading time will vary; my bread machine took about 45 minutes.) Roughly divide the dough in two, then again divide one of the halves in two, so that the pieces are roughly 1/4, 1/4, and 1/2 of the original.
Add sweet potato powder to one 1/4 piece and charcoal powder to the other 1/4 piece. Knead until color is evenly distributed.
Cover all the pieces and let rise for 1-2 hours -- until doubled in volume.
Once risen, divide each dough ball into 10 pieces, slightly varying in size.
Cover the divided dough pieces, and let them rest for 10 minutes.
Roll one purple piece into a long, thin rope (about 18cm long).
Roll one black piece into a flat rectangle or oval (about 8cm x 18cm).
Optionally, sprinkle on some chia seeds.
Place the purple rope at the center of the black dough.
Wrap the sides of the black over the purple. (Due to varying sizes, the black may or may not fully enclose the purple.)
Roll one white dough into a flat rectangle (about 10cm x 20cm).
Place the black-purple rope at the center of the white dough.
Wrap the sides of the white over the purple/black, and pinch to seal the edges.
Place the resulting white log into a greased 8 x 4 inch loaf pan. Repeat steps 8 through 16 with remaining dough pieces. Then, cover the loaf and let it rise for 45-60 minutes. or until nearly doubled in volume.
Bake in a preheated oven at 350F / 180C for 40 minutes, covering the top of the bread with aluminum foil when the top begins to darken in color.
Let the loaf cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.