Have you ever cut into a loaf of walnut bread only to find the inside is purple? According to the Journal of Agriculture and Food Chemistry, this is due to ingredients containing iron (such as certain flours) having a chemical reaction with the gallic acid found in walnut skins. Of course, there are other factors that contribute to the purple hue, including the amount of time the dough sits before baking, and the acidity levels of the other ingredients. For example, baked goods containing yeast have a higher chance of a color reaction due to the acids created by the yeast as it ferments, and the time it takes for the dough to rise.
Try blanching your walnuts in boiling water for one minute prior to baking to eliminate the purple coloring in your bread.
*Special thanks to The Perfect Loaf for use of their photography.
