Controls the rhamnosylation of reddish anthocyanidin-3-O-glucosides, which is the first step in a series of modifications that finally yield magenta or blue/purple coloured anthocyanins (PubMed:8130799, PubMed:8130800). Controls the conversion of anthocyanidin-3-O-glucosides to anthocyanidin-3-O-rutinosides (PubMed:8130800). Pigment biosynthesis; anthocyanin biosynthesis. Expressed in petals, styles and anthers. Expressed at high levels in petals from flowers at an early stage of development, and expression levels decline as the flower matures. Belongs to the UDP-glycosyltransferase family.