Flower colors are generally determined by what kind of pigments accumulate in their petals. The components of flower colors, such as red, purple, and blue, are collectively called anthocyanins. The structures and colors of anthocyanins are varied, but the structures of its backbone moiety controlling coloring (chromophores) can be classified into three major types (Figure 1). Pelargonidin is often contained in orange or bright red flowers, such as salvia, whereas cyanidin is contained in red and showy pink flowers, such as roses. Compounds contained in many purple and blue flowers, such as gentian and platycodon, are delphinidin (also called blue pigment). The structures of these compounds are different only in the number of hydroxyls (OH) (Reference page: Flower colors can be changed freely). This small difference causes a difference in color.
There are roses of various colors, including red, orange, and pink, and these colors are derived from cyanidin and pelargonidin. Red roses primarily contain cyanidin whereas orange roses primarily contain pelargonidin. The reason why there are no blue roses is that no blue pigment (delphinidin) exists in the petal. Some roses created by conventional breeding are collectively called "bluish," but they don't contain delphinidin. Breeding of these varieties was performed by reducing the amount of cyanidin to make them look more bluish. The color of yellow roses is derived from carotenoids (components of the color of pumpkins and carrots), which are a totally different compounds from anthocyanins.
The kind of pigments that plants synthesize is determined by nature, depending on what kind of genes they have. Because roses do not have the genes necessary to synthesize blue pigments (blue genes; scientifically, flavonoid 3',5'-hydroxylase genes [Figure 1]), they do not synthesize them. Therefore, even though hybridization was repeated, roses could not synthesize blue pigments because the rose family does not have blue genes (strawberries and apples, which belong to the rose family, do not have them, either). That was why we could not produce blue roses.
Using biotechnologies, we can change the characteristics of the target living organisms, using genes of other organisms. By introducing blue genes isolated from blue flowers into roses and operating them to work well there, we created roses that accumulated blue pigments. Using various ingenuities, we increased the percentage of blue pigments, and produced roses with blue-colored flowers. Because their colors were more pure blue than those of conventional roses and they accumulated blue pigments (Figure 2), a public announcement was made in 2004 on the birth of blue roses, which was widely reported in Japan and abroad.