Measuring plant colors.

Plant colors such as 'green leaf' and 'red apple' are often described based on human sense, even in scientific papers. On the other hand, colors are measured based on colorimetric principles in some papers, especially in the studies of horticultural plants. The science of color measurements ('colorimetry') is not included in any of the popular lectures in schools and universities, thus the principles of color measurements would not be understood by most researchers. The present review will overview the principles of colorimetry, and will introduce colorimetric methods which can be used for scientific measurement of plant colors. That is to say, the reflection spectrum of visible light (380-780 nm) is measured at 5-nm intervals on the surface of leaves or petals in 'Spectrometric Color Measurement' (SCM). The spectral data is multiplied with RGB or XYZ color matching functions and integrated to obtain RGB or XYZ intensities. Alternatively, approximate RGB values are directly obtained in 'Photographic Color Measurement' (PCM). RGB/XYZ intensities are further calculated to obtain 'hue', 'saturation', and 'lightness', the three factors of colors. Colorimetric insights into genetic regulations (such as MYB gene) and physiological regulations (such as alexandrite effect) of plant colors are also described.