The plant kingdom offers a bewildering variety of flower colors. In my thesis, I investigate the mechanisms and functions of flower coloration. Via a newly developed optical model I show how changes in floral anatomy, e.g. concentration and localisation of pigment as well as thickness of the flower, influence the visual signal of flowers. The visual signals are interpreted with insect vision models to infer how flowers optimize their visibility to pollinators. I also investigate the mechanism and biological relevance of structural coloration and iridescence of flowers. In virtually all cases, the visual signal due to structural coloration and iridescence is negligible compared with the visual signal due to pigmentary coloration. A special case occurs in buttercup flowers, which feature a distinct gloss in addition to their yellow coloration. The gloss of buttercup flowers is due to a truly unique anatomy. The upper (epidermis) layer is very flat and smooth, and, because it is separated from the lower layers by large air spaces, it acts as an optical thin film. In a Dutch nature reserve (the Drentsche Aa) I found that pollinator-competing flowers are more distinctly colored than non-competing flowers. Via a literature review I describe patterns of sexual trait decay in clonal plants. Finally, I discuss why many spectacular visual signals of flowers are generally invisible to flower visitors and should thus be considered a by-product of other traits. This thesis is one of the very few studies with specific focus on the optical mechanisms of flower coloration.