Anthocyanins Give Red Wine Their Color

The third class of Flavonoids Found in Grapes are the anthocyanins. The anthocyanins exist in grapes as the glycoside, which means that the flavonoid portion, or the aglycone is complexed with a sugar.¹ Red wine color comes from the anthocyanins contained in the skins of the grape unless the grape is a variety of teinturier, where even the pulp of the grape is colored. One of the reasons why Pinot Noir is not a very highly colored wine is that it contains only the five anthocyanins shown below, whereas other red varieties contain these 5 anthocyanins as well as anthocyanins that are further modified.
There are 5 basic anthocyanins as shown below:²

In the table below, the structure that is shown is a generalized anthocyanin with R groups that can be modified at R1, R2, R3, R4 or R5 to give the various forms of anthocyanins that can be present in grapes.¹

The substitution pattern on the phenyl ring with R1, R2, and R3 groups affects the color that we perceive. The more hydroxyls (-OH) that are on the phenyl ring, the bluer the hue of the wine. The more methoxy (-OCH₃) substituents on the phenyl ring, the redder the hue of the wine. Since the most prevalent anthocyanin in grapes is malvidin-3-glucoside, the hue of a young wine is red.
References:
1. Jackson, Ronald S., Wine Science: Principles and Applications, Third Edition, Elsevier, Academic Press, 2008, pg. 287. The anthocyanin chart also comes from Ronald Jackson, pg. 287.
2. Douglas Adams, VI257, Lesson 8, pg. 23.
All structures were drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.