Alexandrite-like effect in purple flowers analyzed with newly devised round RGB diagram

The gemstone alexandrite is known for its feature to change color depending on the spectral quality of the incident light. Thus, the stone looks green when illuminated by white LED light but looks red when illuminated by incandescent light. This effect (alexandrite effect) is caused by a special relationship between the spectral quality of the incident light and the absorbance spectrum of the stone. Here we report an alexandrite-like effect in the petals of torenia and cyclamen flowers. These flowers are purple in sunlight but magenta (reddish) in incandescent light, and violet (bluish purple) in white LED light. The m-n, triangle and round diagrams are devised to calculate the colors of visible light spectra, based on the RGB color-matching function. Using these calculations, the alexandrite-like effect in purple flowers was successfully analyzed in terms of the interaction between the incident light spectrum and the absorbance spectrum of their purple anthocyanin. This analysis allows both logical and intuitive understanding of the colors exhibited by any object showing alexandrite-like properties.