Transfers hydroxycinnamic moieties to the glucosyl groups of anthocyanin. (E)-4-coumaroyl-CoA + an anthocyanidin 3,5-di-O-beta-D-glucoside = an anthocyanidin 3-O-beta-D-glucoside 5-O-beta-D-[(6-O-(E)-4-coumaroyl)glucoside] + CoA Activity enhanced by manganese ions and inhibited by p-chloromercuribenzoate. Expressed in the outer epidermal cells of petals, but not in sepals, leaves or stems. Induced during petal development and then decreases rapidly with maturation of the flower. The N-terminus is blocked. Cannot use malonyl-CoA as a donor. Belongs to the plant acyltransferase family.