Why is the apple red?
The skin builds red pigments that absorb most colors and bounce red light back at your eye.
Color is light that bounces back
An apple does not contain redness the way a bucket contains water. Daylight is a mixture of every visible color, and the skin soaks up most of those colors. The wavelengths it does not absorb scatter back out, and red is what reaches you.
The pigment behind the red
The red comes from anthocyanins, water-soluble pigments stored in the outer cells of the skin. They are the same family of molecules that paint red cabbage, cherries, and autumn leaves. The greener an apple is, the more it is still dominated by chlorophyll instead.
Anthocyanins also shift with acidity, which is why the exact shade ranges from crimson to a darker burgundy across varieties.
Why apples turn red as they ripen
The color is also an invitation. A vivid red signals ripe, sugary flesh to birds and mammals, who eat the fruit and carry the seeds somewhere new. The apple is, in a sense, advertising.