Is it true that only white bread contains yeast, and dark bread contains sourdough?

No, this assumption is not true. Yeast and sourdough do not determine the colour of the bread. Yeast is essential in the fermentation process of bread, whether this is white or brown bread. Yeast provides the typical alveolated structure, softness, taste and smell of wheat bread. Both yeast and sourdough give a specific flavour, taste and texture to the final baked product whether white and dark bread. Even if not added on purpose, sourdough always contains yeast.