Veraison is the beginning of the ripening process of grapes when they change colours and start to sweet naturally. The grapes lose their dark green colour, turning golden for white grapes and red/purple for blue grapes. This process in the grapes happens through photosynthesis.
The vines move their energy from the roots to the grapes leading to an increase in size and sugar levels. The acids will decrease as the sugars rise, until the grapes are perfectly balanced and ready to harvest. Each variety ripens in its own unique way, with some ripening more evenly than others.
The outcome of veraison is also a result of the summer pruning.
The sun is required for veraison, but the leaves also protect the grapes from overexposure which is cared for during pruning. Vineyard management is an interesting procedure since it represents a shift in how winemakers care for their vines which can be tasted back in your glass of wine.