Colour: where it comes from and how it changes

Almost all grape juice is pale regardless of skin colour. Colour is therefore a record of what was done in the winery as much as of what was grown.

The exceptions are teinturier varieties such as Alicante Bouschet, which have coloured flesh as well as coloured skins.

Colour is genuinely informative about age and about production, and much less informative about quality than it is usually taken to be.

Anthocyanins live in the grape skin and are extracted into the wine during maceration. How much is extracted depends on skin contact time, temperature, alcohol level and how the cap is worked.

Anthocyanin colour is pH-sensitive: the pigments are more intensely red at lower pH, which is one reason a high-acid red can look brighter than a low-acid one of the same age.

Free anthocyanins are unstable on their own. During ageing they react with tannins to form polymeric pigments, which are more stable and less purple. This is the shift from purple-red through ruby to garnet and brick.

Eventually the polymers grow large enough to fall out of solution, which is the sediment in an old red and also the reason very old reds can look pale and watery at the rim.

White wine has few anthocyanins and its colour change runs the other way. Oxidation of phenolic compounds produces yellow and brown pigments, so whites deepen from near-colourless through lemon and gold towards amber.

Orange wine sits in between: white grapes fermented on their skins pick up phenolics and pigments that white winemaking normally removes.

A rim that has gone brick-orange is a genuine indication of age in a red, and a white that has gone deep gold when it should be pale is a reasonable warning of oxidation.

It also explains why rosé colour is a production decision rather than a quality signal — pale Provençal rosé is pale because of how briefly the juice met the skins.