I’m hoping you’re looking for the scientific answer, in which case I think I might be able to help you. Essentially, although we only see chlorophyll (the green pigment in leaves), there are other pigments. These pigments are called accessory pigments. Chlorophyll is the dominant pigment, which is why we do not see these other pigments. However, colder temperatures destroy chlorophyll and break it down. Chlorophyll is constantly break down and being replaced but, the cold temperatures slow down this process, allowing the accessory pigments to show through. Hope this helps!