## 1 Introduction  
Anthocyanins are flavonoid pigments of great interest as they provide color and possess antioxidant properties [1]. Foods of economic importance rich in anthocyanins include red grapes and derived beverages such as red wines in which color is one of the most important quality parameters that plays a key role in the acceptability of consumers. The color of anthocyanins is the result of the resonant structure of the flavylium chromophore, which is composed of two aromatic rings (A: benzopyriliium ring, B: phenolic ring) linked by a chain of three carbon atoms (Figure 1). This chromophore undergoes reversible structural transformations in aqueous solutions with pH modifications, consisting in a variety of species having different colorations [2]. Under acidic conditions (pH < 3), the colored flavylium cation is the predominant form of anthocyanins, and it loses its vivid coloration at near-neutral pH values. The color (red, orange, purple, or blue), intensity, and stability of the flavylium cation are highly variable depending on its chemical structure [3] and environmental factors such as interactions with other compounds [4, 5]. The most common anthocyanins described in fruits, vegetables, and derived food products are 3-glucosidase of the anthocyanins cyanidin, delphinidin, malvidin, peonidin, petunidin, and pelargonidin. Aglycones differ from each other in the methoxyl and hydroxyl substitution pattern of ring B and in the sugar substitutions and acylation (number, type, and position in the aglycone skeleton). The small conformational distortions of the aglycone (stretching, bending, or torsion) related to the molecular substitutions modify the \(\pi\)-delocalization of the chromophore, resulting in perceptible changes in hues [5; 6]. Anthocyanins are highly unstable but can interact with other compounds, affecting their color expression and stability through the copigmentation phenomenon. The copigmentation phenomenon consists of non-covalent interactions between the planar polarizable nuclei of anthocyanins' colored forms (mainly the flavylium cation) and diverse colorless organic molecules (copigments) forming molecular complexes [7]. Biopolymers such as polysaccharides or proteins have been demonstrated to modulate the color of anthocyanins such as malvidin 3-glucoside (mb3 glc) by weak molecular forces (hydrogen bond interaction and hydrophobic effects such as \(\pi\)-\(\pi\) stacking) similar to copigmentation [8; 9], but sometimes unpredictably [10]. Thus, the use of proteins from different plants and agricultural by-products as anthocyanin stabilizers has been studied with great interest [11]. Accordingly, grape seed globulins interact with mv3 glc through a static binding mechanism, contributing to positively modulating its color, which has been confirmed by fluorescence techniques and colorimetric investigation [12]. Furthermore, soluble protein hydrolysates or peptides obtained through enzymatic methods or by separation techniques may harbor similar applications [13]. Peptides rich in aromatic or hydrophobic amino acids could establish interactions with anthocyanins, playing a role in their chemical stability and color expression [14; 15]. UV-vis spectroscopy is the most common tool used to study the spectral changes of anthocyanins due to variations in their conformation, chemical structure, or molecular interactions. The colorimetric behavior of the anthocyanin pigments can be assessed with an accurate description of the bands over the whole UV-vis range (360-780 nm) and by applying the basis of tristimulus colorimetry [16; 17]. The perceptually uniform CIE 1976  
Figure 1: Three-dimensional structure of flavylium chromophore, indicating the notation of the aromatic rings and the dihedral angle. (\(L^{*}a^{*}b^{*}\)) color space (CIELAB), based on tristimulus colorimetry, is the most recommended for color specifications [18].