In melittophilous plants the colour pattern of the flowers, as perceived by bumblebees, is a gradient of centripetally increasing spectral purity. This pattern serves as a signal for innate flower recognition in naive bumblebees permitting orientation to flowers and landing on flowers. Structures which make up the total signal pattern can include the background (e.g., green leaves), corollas, and stamens or floral guides. How various colour parameters, such as dominant wavelength, intensity, and spectral purity influence the colour signal pattern of flowers is analyzed. The process of strong absorption of ultraviolet light is shown to be a mechanism for the enhancement of spectral purity in flower guides. The importance of other mechanisms is also demonstrated. The presence of a gradient of centripetally increasing spectral purity in floral colour patterns as perceived by a bumblebee''s eyes is demonstrated by a comparison of the spectral reflectance in different parts of the flower and a representation of colour loci in the colour triangle.