Blue flower colors are primarily due to anthocyanin, a flavonoid pigment. Anthocyanin itself is purple in neutral aqueous solutions, ans its color is very unstable and quickly fades. Therefore, the mechanism of blue color development in living flower petals is one of the most intriguing problems in natural product chemistry. Much progress has been made in understanding blue flower coloration since the comprehensive review by Goto and Kondo in 1991. This review focuses on the advances in the last 15 years, and cites 149 references.

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;Sepal color of hydrangea varies with the environmental conditions. Although chemical and biological studies on this color variation have a long history, little correct knowledge has been generated about color development. All colored sepals contain the same anthocyanin, delphinidin 3-glucoside. Thus, there must be some other system for developing the wide variety of colors. In hydrangea sepals the cells of the epidermis are colorless and only the second layer of cells contain pigment. We prepared protoplasts without any color change during enzyme treatment of sepals and measured the vacuolar pH of each of the colored cells. We could correlate the color of a single hydrangea cell with its vacuolar pH using a combination of micro-spectrophotometry and a proton-selective microelectrode. Values for the vacuolar pH of blue (lvismax: 589 nm) and red cells (lvismax: 537 nm) were 4.1 and 3.3, respectively, the vacuolar pH of blue cells being significantly higher.

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