I made green tea muffins following the recipe here. I like my muffins moist and fluffy, so I added banana as suggested by the recipe and replaced half of the baking powder with baking soda. I also added a teaspoon of rice vinegar to activate the baking soda.
Right before baking the mix was vibrantly green, but after 13 minutes in the oven the muffins had turned very dark — and this morning they were even darker.
What happened, and how can I prevent it?