Why did my baos turn brown when steaming them?

I used plain flour and followed the recipe (as per this book https://www.phaidon.com/store/cookbooks-food-and-drink/bao-9781838666200/ no weird ingredients ) and before steaming the dough was white! I only have made bags one other time years ago and they stayed white after steaming.

I uploaded some images here: https://imgur.com/a/QkKqWNP