Blue flower colour was always associated with something unusual and desired. Blue roses especially were assumed to be a dream that cannot be realised. Blue colour in flower petals is caused by anthocyanins, which are members of flavonoid class metabolites. We can diversify three main classes of anthocyanin pigments: cyaniding type (two hydroxyl groups in the B-ring) responsible for red coloration, pelargonidin type (one hydroxyl group in the B-ring) responsible for orange colour and delphinidin type (three hydroxyl groups in the B-ring) responsible for violet/blue flower and fruits coloration. The main difference in the structure of listed anthocyanins type is the number of hydroxyl groups in the B-ring of the anthocyanin. Nevertheless, in the monomeric state anthocyanins never show blue colour in the weak acidic and neutral pH. The mechanism of blue colour formation are very complicated in most cases, presence of delphinidin type pigments is not sufficient, great role play also the pH and the formation of complexes of anthocyanins with flavones and metal ions.