Flavanols are "sunscreen" for grapes. They have an effect on co-pigmentation. Michael Garrison, 2015.
Flavonols are a class of flavonoids that tend not to receive as much attention as the other flavonoids such as tannins and anthocyanins; however, flavonols can increase the color intensity by several fold through copigmentation, particularly in younger red wine. Also, the amount of flavonols in a wine is highly correlated with its market value. High value red wine tends to have three to four times the amount of flavonols as low value wine (Ritchey, 1999). Copigmentation occurs when the flavonols complex with the colored flavylium ion form of anthocyanin. The formation of these complexes push the anthocyanin equilibrium to produce more of the colored flavylium form. This can cause up to a tenfold increase in color absorbance as well as a bathochromic shift in color by about 5-20 nm which makes the wine appear more purple. (Boulton, 2001). These effects are diminished as the wine ages and the complexes are broken up as polymerized pigments are formed.
The flavonol content of the finished wine is a function of how much was produced in the berries. The biosynthetic pathway for flavonols is stimulated by UV light exposure. It has been shown that exposing the clusters to light via any viticultural practice, such as leaf pulling, shoot thinning, or canopy management through deficit irrigation, has resulted in increased flavonol biosynthesis both pre- and post-veraison (Sternad, 2013). However, elevated temperatures may inhibit flavonol biosynthesis, but this has not been quantitatively established yet. Each cultivar also produces varying amounts of total flavonols as well as different proportions of the different flavonol compounds (Castillo-Munoz, 2007).
References:
Boulton, R. 2001. The copigmentation of anthocyanins and its role in the colour of red wine: a critical review. Am. J. Enol. Vit. 52:67-87.
Castillo-Munoz, N., S. Gomez-Alonso, E. Garcia-Romero, and I. Hermosin-Gutierrez. 2007. Flavonol profiles of Vitis vinifera red grapes and their single-cultivar wines. J. Agric. Food Chem. 55: 992-1002.
Ritchey, J.G. and A.L. Waterhouse. 1999. A standard red wine: monomeric phenolic analysis of commercial cabernet sauvignon wines. Amer. J. Enol. Vit. 50: 91-100.
Sternad Lemut, M., K. Trost, P. Sivilotti, P. Arapitsas, and U. Vrhovsek. 2013. Early versus late leaf removal strategies for Pinot Noir (Vitis vinifera L.): effect on colour-related phenolics in young wines following alcoholic fermentation. J. Sci. Food Agric. 93: 3670-3681.