The yellow color comes from the tumeric. "Turmeric comes from the root of the Curcuma longa plant and has a tough brown skin and a deep orange flesh.... Turmeric has a peppery, warm and bitter flavor and a mild fragrance slightly reminiscent of orange and ginger, and while it is best known as one of the ingredients used to make curry, it also gives ballpark mustard its bright yellow color.... " It is also thought to be one of the world's healthiest foods.
Although the original recipe only had you waiting overnight before eating, I would that the texture of the zucchini was still too firm at that point and did not have the texture of a pickle. After about 2 days the flavors and texture was wonderful.
Bread and Butter Zucchini Pickles
adapted from Food & Wine August 2014
1/2 pound firm medium zucchini, sliced into 1/8 inch thick discs
1/4 cup thinly sliced onion
1 1/2 Tbsp kosher salt
3/4 cup apple cider vinegar
2 1/2 Tbsp sugar
1 tsp mustard seeds, crushed
3/4 tsp dry mustard powder
1/2 tsp ground turmeric
1/2 cup water
Toss the zucchini and onion with 1 Tbsp of kosher salt in a large bowl. Cover with ice water and let stand until just softened, about 45 minutes. Drain the zucchini and onion well and pat dry.
Combine the remaining 1/2 Tbsp salt with the vinegar, sugar, mustard seeds, mustard powder, turmeric and 1/2 cup of water. Bring to a boil, stirring to dissolve the sugar; set aside to cool completely.
Place the zucchini and onion in a 2 cup container and pour in enough brine to cover. Seal the container and refrigerate for 2 days before using.