Shades of Brown

They can leach a purplish colour and depending on what flour you're using can cause this effect. Doesn't make it any less tasty though. Although toasting them before adding to the dough has even nicer results. 

As for over proofing it's difficult to tell from the photo. It seems to have bloomed nicely so can't be that over proofed if it is at all.

According to Laurel's Kitchen Bread Book (p. 223), walnuts tend to color the loaf a sort of lavender-gray unless they are added just before shaping. Just putting that out there as a possibility.

Edit: Cross-posted.

Hi Louis,

I was blocked by a log-in window and couldn't see your photos, but I'll trust the others on their assessment about the walnuts. It reminded me that Cook's Illustrated wrote about this phenomenon a few years ago:

"Theories abound on why yeast breads that contain walnuts (or pecans) turn purple. After reading a paper in the Journal of Agricultural and Food Chemistry, we decided to pursue the one that seemed most plausible: that the iron in flour is reacting with gallic acid found in walnut skins to create a color change.

To confirm this theory, we combined walnuts, water, and (to minimize variables) crushed-up iron supplements from the vitamin aisle and watched the water immediately turn a deep purple. We repeated the experiment with pecans, which were equally reactive, and almonds and hazelnuts, both of which triggered a slightly less dramatic color change.

So why don’t all baked goods containing these nuts turn purple? The answer has to do with acidity and time. The purple tint appears only in an acidic environment, and most baked goods aren’t acidic enough to cause the reaction. But thanks to the acids produced by yeast as it ferments, bread dough does have what it takes. In addition, since gallic acid leaches out slowly, rising bread dough gives time for the effect to happen.

Only tiny amounts of gallic acid are needed to bring out the purple. The tint can’t be prevented completely, but it can be minimized by blanching the walnuts in boiling water for 1 minute before using."

So, time, acidity, composition of the nuts and the flour --- any or all of these things can vary from bake to bake. If you do give the blanching a try, you can follow that by toasting if you like.

My best,
dw