The reason why the mooncake skin turns yellow is because too much alkaline water has been added.

In the process of making Cantonese-style mooncakes, alkaline water is one of the essential ingredients. Usually, you can use edible soda ash and water in a ratio of 1:3, or use 1 gram of baking soda and 10 grams of water to prepare alkaline water. The yellowing of the mooncake skin is caused by an improper ratio of alkaline water to inverted syrup, or excessive use of alkaline water. When making mooncake skin, low-gluten flour, salad oil, syrup, alkaline water and salt are usually mixed and evenly kneaded into a dough, then covered with plastic wrap and left to rest for 1 hour. If too much alkaline water is added, the mooncake skin will turn yellow.