Add the sourdough starter to the warm water and whisk to combine.
Add the bread flour gradually to the water mixture and stir with a dough whisk until the flour is moistened and you have a shaggy dough. Cover the dough and set it aside for 20 minutes.
Pull the dough from one edge in an upward motion and then fold it over the rest of the dough. Turn the bowl and repeat again. Complete 4 turns.
Repeat the dough turns after another 20 minutes. Then repeat again after another 20 minutes. Then after a third 20 minutes. Add the salt.
Melt the white chocolate in 30-second increments in a microwave-safe bowl. Stir in the sugar.
Separate the melted chocolate into 6 bowls and mix in a few drops of food coloring to tint each one a different color from the rainbow.
Section the dough into 6 equal-sized pieces and combine each piece with a different colored chocolate.
Cover the bowls and let the dough rise on the counter for 4-6 hours.
Turn the red dough onto a clean lightly floured surface and stretch to form a 5 x 7-inch rectangle.
Repeat with each piece of colored dough, stacking each on top of the other in rainbow order.
Turn the dough over and fold the sides into itself like an envelope then form the dough into a ball.
Flip the dough and place it in a towel-lined basket. Place in the refrigerator overnight.
Preheat the oven to 450 F and place a large Dutch oven with a lid inside.
Flip the dough out of the basket onto a piece of parchment paper and score the top of the loaf.
Remove the Dutch oven from the oven, place the bread inside, and cover with the lid.
Bake for 20 minutes, then remove the lid and continue baking for another 20 minutes.