Apples are red primarily due to the presence of pigments called anthocyanins. These pigments develop in the skin of the apple as it ripens, influenced by factors such as:

1. Genetics: Different apple varieties have varying levels of anthocyanins, which determine their color.

2. Sunlight: Exposure to sunlight can enhance the production of these pigments, leading to deeper red hues.

3. Temperature: Cooler temperatures during the ripening process can also increase anthocyanin production.

4. Ripeness: As apples mature, their color changes, often transitioning from green to red.

In summary, the red color of apples is a combination of genetic factors and environmental influences that promote the synthesis of anthocyanins.