Colour changes in the petals of two varieties of the tropical plant species Malvaviscus arboreus were investigated. The light absorbance of living petals and the absorbance of petal-extracts were measured. A separation of pigments by means of paper chromatography was attempted. Older stages of living petals show a low absorbance at all wavelengths compared with younger ones, and also a shift of the minimum at 440 nm and the maximum at 520 nm towards higher wavelengths. Petal extracts of different stages also have a different absorption spectrum; nonnectar producing flowers show a maximum at about 515 nm while nectar producing ones have merely a shoulder at 515 nm which may be due to a loss of red pigments. This loss is clearly demonstrated by paper chromatography and coincides with the beginning of nectar production. Hummingbirds, the main visitors of Malvaviscus, probably use this colour-shift for recognizing nectar producing flowers.