The economic value of cut flowers is considerable. Because novelty is a driving force in this section of horticulture, the development of new flower varieties is of great interest. In this connection new colours are particularly important. Classical plant breeding has been able to produce a broad variety of colours and forms for flowers but with the tools for genetic engineering available, a new era for cut flower plant breeding has dawned. The present study reviews the various aspects connected with the molecular engineering of flower colours. The results achieved so far, bottlenecks and possible approaches in altering flower colours are discussed. The methods available for genetic modification of plants are briefly reviewed. The study concentrates on the anthocyanins. The biosynthetic pathway leading to these compounds is extensively described, as well as knowledge of its regulation at molecular level. The genes involved in the regulation are reviewed. The colours connected with the individual compounds are considered, as is the influence of chemical modifications of anthocyanins, such as acylation of sugar residues. However, flower colour is not only determined by the anthocyanins present: co-pigmentation also plays an important role in stabilizing and altering colours. Compounds involved in co-pigmentation and other conditions involved (such as pH) are discussed, as well the mechanisms behind this phenomenon. The study includes an extensive literature search on the constituents of the pigments of the major cut flowers. Tables are given in which the constituents of the major cut flowers are summarized. More than 600 references are cited. Based on present knowledge, the strategy of engineering a certain colour in a specific species can be developed. Possible approaches for genetic engineering are discussed for the major colours.