Redness in apple is due to

Redness in the apple is due to Anthocyanin.
Anthocyanins are coloured water-soluble pigments belonging to the phenolic group. The pigments are in glycosylated forms.
Anthocyanins responsible for the colours, red, purple and blue, are in fruits and vegetables. Berries, currants, grapes, apples and some tropical fruits have high anthocyanin content.
One of the most common anthocyanin pigments is cyanidin, which, in the form of cyanidin 3-O-galactoside, is the pigment primarily responsible for red colouration in apple skin.