A quantitative photographic survey was made of the spectral structure of the colours of a random selection of wild and cultivated flowers, in the insect and human visible spectra. The survey revealed that the majority of insect-pollinated flowers possessed nectar guides, of which three types were recognized. Analysis of the contrasts between the nectar guides and the petals revealed that they were apparently most conspicuous in the insect-visible spectrum. Cultivars were found to have a significantly lower incidence of nectar guides and were also apparently more conspicuous in the human visible spectrum. These results were discussed in relation to possible floral colour adaptation to insect pollinator attraction.