Anthocyanins

Anthocyanins are phenolic compounds located in the skin cells of red and black grape varieties (but not in the pulp). This is why white wines can be made from red grapes — if the juice is separated from skins quickly, it remains colourless, as in Blanc de Noirs Champagne.

Anthocyanin content varies by variety. Deeply coloured grapes such as Syrah, Malbec, and Sagrantino are rich in anthocyanins; lighter-coloured varieties like Pinot Noir and Grenache contain significantly less, which partly explains their paler wine colour.

Anthocyanins are unstable molecules. In young red wines they exist as free, brightly coloured compounds, giving deep purple-ruby hues. Over time they react with tannins to form larger, stable polymeric pigments (co-pigmentation). These polymerised pigments are more orange-garnet-brick in colour and less vivid, explaining why aged red wines shift in colour from the rim.

For the exam, anthocyanins are the reason red wine is red, why Pinot Noir is paler than Malbec, and why old red wines show garnet and brick colours at the rim.